PAPERS · ISRO

ISRO Scientist/Engineer Computer Science 2025 question paper

All 95 questions of ISRO's written test for Scientist/Engineer 'SC' (Computer Science), held on 26 October 2025, with ISRO's answer key. ISRO excluded Q.3, Q.50, Q.75 and Q.82 from evaluation.

Answers from ISRO's final answer key of 15 May 2026.

The question paper (PDF, ISRO)ISRO’s answer key (PDF)

questions · 100 marks: 95questions · 100 marks
Part A: Computer Science: 80Part A: Computer Science
Part B: aptitude: 15Part B: aptitude

How the paper is marked

Part A (Q.1 to Q.80): 1 mark for a correct answer, 0 for no answer, minus 0.33 for a wrong answer.
Part B (Q.81 to Q.95): 1 or 2 marks as printed with each question, 20 marks in all, no negative marking.

Source: the instructions printed in the question paper.

ISRO CS 2025 answer key

ISRO's answer key for ISRO Scientist/Engineer Computer Science 2025
Question	Part	Marks	Answer
Q.1	A	1	C
Q.2	A	1	D
Q.3	A	1	Excluded
Q.4	A	1	B
Q.5	A	1	A
Q.6	A	1	A
Q.7	A	1	C
Q.8	A	1	A
Q.9	A	1	B
Q.10	A	1	B
Q.11	A	1	C
Q.12	A	1	C
Q.13	A	1	A or B
Q.14	A	1	D
Q.15	A	1	B
Q.16	A	1	B
Q.17	A	1	D
Q.18	A	1	C
Q.19	A	1	C or D
Q.20	A	1	A or C
Q.21	A	1	A
Q.22	A	1	B
Q.23	A	1	B
Q.24	A	1	D
Q.25	A	1	B
Q.26	A	1	C
Q.27	A	1	A
Q.28	A	1	A
Q.29	A	1	A
Q.30	A	1	B
Q.31	A	1	B
Q.32	A	1	A
Q.33	A	1	B
Q.34	A	1	C
Q.35	A	1	C
Q.36	A	1	C
Q.37	A	1	D
Q.38	A	1	A
Q.39	A	1	B
Q.40	A	1	D
Q.41	A	1	C
Q.42	A	1	D
Q.43	A	1	B
Q.44	A	1	A
Q.45	A	1	B
Q.46	A	1	D
Q.47	A	1	A
Q.48	A	1	D
Q.49	A	1	B
Q.50	A	1	Excluded
Q.51	A	1	C
Q.52	A	1	A
Q.53	A	1	C
Q.54	A	1	A or D
Q.55	A	1	C
Q.56	A	1	D
Q.57	A	1	C
Q.58	A	1	D
Q.59	A	1	C
Q.60	A	1	B
Q.61	A	1	D
Q.62	A	1	D
Q.63	A	1	C
Q.64	A	1	A
Q.65	A	1	D
Q.66	A	1	A
Q.67	A	1	C
Q.68	A	1	C
Q.69	A	1	C
Q.70	A	1	D
Q.71	A	1	A
Q.72	A	1	B or C
Q.73	A	1	C
Q.74	A	1	D
Q.75	A	1	Excluded
Q.76	A	1	A
Q.77	A	1	D
Q.78	A	1	C
Q.79	A	1	B
Q.80	A	1	A
Q.81	B	1	D
Q.82	B	1	Excluded
Q.83	B	2	B
Q.84	B	1	C
Q.85	B	1	B
Q.86	B	2	B
Q.87	B	1	D
Q.88	B	1	C
Q.89	B	2	D
Q.90	B	1	C
Q.91	B	1	B
Q.92	B	2	D
Q.93	B	1	D
Q.94	B	1	C
Q.95	B	2	A

ISRO excluded Q.3, Q.50, Q.75 and Q.82 from evaluation.

Part A: Computer Science Q.1 to Q.80

Q.11 markOfficial answer: C
Tap to open full size
Data Structures and Algorithms › Trees & heaps
Inorder means left subtree, node, right subtree, applied recursively. A node with only a left child is visited right after that child's subtree.
Full solution
Q.21 markOfficial answer: D
Tap to open full size
Data Structures and Algorithms › Design techniques
Huffman's greedy algorithm repeatedly merges the two lowest-frequency nodes, so symbol frequencies alone decide the code lengths and the tree shape.
Full solution
Q.31 markExcluded from evaluation by ISRO
Tap to open full size
Earlier, ISRO's key gave A.
Q.41 markOfficial answer: B
Tap to open full size
Data Structures and Algorithms › Design techniques
A greedy algorithm commits at every step to the locally best choice under a fixed criterion and never revisits it. It is an optimization technique, but its correctness must be proved for each problem.
Full solution
Q.51 markOfficial answer: A
Tap to open full size
Earlier, ISRO's key gave D.
Discrete Mathematics › Graph theory
Malls are vertices and roads are edges, so the network is a connected planar 3-regular graph. The handshake lemma gives $E = 3 V /2 = 30$ , and Euler's formula $V - E + F = 2$ gives the number of regions the roads mark off, $F = 12$ , with one park per region. The dodecahedron, which has 20 corners of degree 3 and 12 faces, is a concrete example.
Full solution
Q.61 markOfficial answer: A
Tap to open full size
Earlier, ISRO's key gave B.
Discrete Mathematics › Graph theory
A triangle forces at least 3 colours. Then show a proper 3-colouring exists by colouring the triangle first and extending vertex by vertex, which settles the chromatic number as exactly 3.
Full solution
Q.71 markOfficial answer: C
Tap to open full size
Earlier, ISRO's key gave no answer (excluded).
Data Structures and Algorithms › Sorting & hashing
After $k$ iterations insertion sort has the first $k + 1$ elements sorted and the rest untouched, while selection sort has the $k$ smallest elements in final place and bubble sort has the $k$ largest at the end.
Full solution
Q.81 markOfficial answer: A
Tap to open full size
Discrete Mathematics › Counting
A symmetric cipher needs one shared key per unordered pair of users, so $n$ persons need $(2 n) = \frac{n ( n - 1 )}{2}$ keys.
Full solution
Q.91 markOfficial answer: B
Tap to open full size
Q.101 markOfficial answer: B
Tap to open full size
Q.111 markOfficial answer: C
Tap to open full size
Q.121 markOfficial answer: C
Tap to open full size
Q.131 markOfficial answer: A or B
Tap to open full size
Earlier, ISRO's key gave A.
Q.141 markOfficial answer: D
Tap to open full size
Q.151 markOfficial answer: B
Tap to open full size
Q.161 markOfficial answer: B
Tap to open full size
Q.171 markOfficial answer: D
Tap to open full size
Q.181 markOfficial answer: C
Tap to open full size
Machine Learning › LDA, SVM & trees
A decision tree is a method, not a task. Class labels at the leaves give a classification tree; numeric leaf values give a regression tree.
Full solution
Q.191 markOfficial answer: C or D
Tap to open full size
Earlier, ISRO's key gave D.
Q.201 markOfficial answer: A or C
Tap to open full size
Earlier, ISRO's key gave C.
Artificial Intelligence › Search
Hill climbing is the standard local search and optimization method in AI: move to the best neighbouring state until no neighbour is better. It is a greedy local strategy, so a bare "greedy algorithm" can also be read as a search/optimization approach.
Full solution
Q.211 markOfficial answer: A
Tap to open full size
Q.221 markOfficial answer: B
Tap to open full size
Q.231 markOfficial answer: B
Tap to open full size
Q.241 markOfficial answer: D
Tap to open full size
Q.251 markOfficial answer: B
Tap to open full size
Q.261 markOfficial answer: C
Tap to open full size
Q.271 markOfficial answer: A
Tap to open full size
Database Management and Warehousing › Data transformation
Classify data by how rigid its schema is. Structured data has a fixed format of rows and columns. Semi-structured data is self-describing through tags or keys (XML, JSON). Unstructured data has no predefined model (free text, images, video).
Full solution
Q.281 markOfficial answer: A
Tap to open full size
Theory of Computation › Finite automata
Read a regular expression piece by piece: literal prefix, character class (case-sensitive), then the quantifier + meaning at least one character.
Full solution
Q.291 markOfficial answer: A
Tap to open full size
Q.301 markOfficial answer: B
Tap to open full size
Compiler Design › Lexical analysis
Phases are chained: each phase's output is the next phase's input. The lexer outputs tokens, and the parser is the next phase, so it receives them.
Full solution
Q.311 markOfficial answer: B
Tap to open full size
Compiler Design › Parsing
yacc turns a grammar written in BNF-style rules into a parser, while lex turns regular expressions into a scanner. The two are normally used together: lex supplies tokens and yacc parses them.
Full solution
Q.321 markOfficial answer: A
Tap to open full size
Theory of Computation › CFG & PDA
Parse trees, leftmost derivations and rightmost derivations correspond one to one. A string is ambiguous exactly when it has two parse trees, two leftmost derivations or two rightmost derivations.
Full solution
Q.331 markOfficial answer: B
Tap to open full size
Theory of Computation › CFG & PDA
Work out the language of each non-terminal separately. $A$ gives $a^{n} c b^{n}$ and $B$ gives $b^{n} e d^{n}$ . Then check each string against the two forms.
Full solution
Q.341 markOfficial answer: C
Tap to open full size
Programming in C › Pointers & arrays
A block from new int [n] has valid indices 0 to n-1. Moving a pointer inside the block is harmless, but reading or writing at index n is an out-of-bounds access with undefined behaviour, and it may crash.
Full solution
Q.351 markOfficial answer: C
Tap to open full size
Computer Networks › Layering & switching
The default gateway is an IP-level routing setting: it tells a host where to send packets for destinations outside its subnet. IP addressing and routing are Layer 3 functions.
Full solution
Q.361 markOfficial answer: C
Tap to open full size
Q.371 markOfficial answer: D
Tap to open full size
Computer Networks › Data link & MAC
Nearest-codeword decoding corrects an error only when exactly one codeword is closest to the received word. A code with $d_{m i n} = 2$ detects single-bit errors but cannot correct them.
Full solution
Q.381 markOfficial answer: A
Tap to open full size
Q.391 markOfficial answer: B
Tap to open full size
Computer Networks › IPv4
Path MTU discovery relies on ICMP 'Destination Unreachable, Fragmentation Needed' messages from routers; TCP's slow start and congestion control rely only on ACKs, timeouts and duplicate ACKs.
Full solution
Q.401 markOfficial answer: D
Tap to open full size
Computer Networks › TCP
The window that keeps a link fully used equals the bandwidth-delay product: the amount of data the link can carry during one delay period, converted from bits to bytes.
Full solution
Q.411 markOfficial answer: C
Tap to open full size
Computer Networks › IPv4
Convert the mask to a prefix length, find the block size in the interesting octet ( $256 - 192 = 64$ ), and see which block each address falls in. Hosts in different blocks need a router.
Full solution
Q.421 markOfficial answer: D
Tap to open full size
Q.431 markOfficial answer: B
Tap to open full size
Database Management and Warehousing › SQL
A view is a stored query, a virtual table. Granting users access to a view that exposes only chosen columns or rows is the standard way to hide sensitive data in the base table.
Full solution
Q.441 markOfficial answer: A
Tap to open full size
Database Management and Warehousing › Normal forms
In a one-to-many relationship, the foreign key sits on the "many" side and points to the primary key of the "one" side.
Full solution
Q.451 markOfficial answer: B
Tap to open full size
Database Management and Warehousing › Indexing
A multi-column index is ordered by its first column, then the second, then the third, so it helps only when the query's conditions cover a leftmost prefix of the index columns.
Full solution
Q.461 markOfficial answer: D
Tap to open full size
Database Management and Warehousing › ER model
One-to-many relationships fold into the 'many' side as a foreign key. Many-to-many relationships need their own table. Entities each get a table.
Full solution
Q.471 markOfficial answer: A
Tap to open full size
Earlier, ISRO's key gave D.
Q.481 markOfficial answer: D
Tap to open full size
Database Management and Warehousing › Normal forms
Find the candidate key by closure, then look for a non-prime attribute determined by part of the key. Such a partial dependency breaks 2NF, and so breaks 3NF and BCNF too.
Full solution
Q.491 markOfficial answer: B
Tap to open full size
Digital Electronics › Number systems
Convert the binary fraction to decimal by adding the weights $2^{- i}$ of the 1 bits, then read off the required digits.
Full solution
Q.501 markExcluded from evaluation by ISRO
Tap to open full size
Earlier, ISRO's key gave C.
Q.511 markOfficial answer: C
Tap to open full size
Digital Electronics › Number systems
Decode each 2's complement number to its signed value, multiply as ordinary integers, check the result fits in 8 bits, then encode it back as $2^{8}$ plus the negative value.
Full solution
Q.521 markOfficial answer: A
Tap to open full size
Digital Electronics › Boolean logic
Convert each product term to its binary cell (complemented variable = 0), then read the map labels carefully, including the order of the column headings, to find the map with 1s only in those cells.
Full solution
Q.531 markOfficial answer: C
Tap to open full size
Digital Electronics › Sequential
A MOD- $N$ up counter in state $S$ needs $N - S$ clock pulses to reach $0$ , because it wraps from $N - 1$ to $0$ .
Full solution
Q.541 markOfficial answer: A or D
Tap to open full size
Earlier, ISRO's key gave D.
Digital Electronics › Sequential
A gated D element is transparent only while the clock (gate) is at its enabling level, high here, and holds its stored bit when the clock is low.
Full solution
Q.551 markOfficial answer: C
Tap to open full size
Digital Electronics › Boolean logic
A NAND gate gives $0$ only when all its inputs are $1$ . Work backwards: for $Y = 0$ , the NOR output must be $1$ (so $A = B = 0$ ) and $C$ must be $1$ .
Full solution
Q.561 markOfficial answer: D
Tap to open full size
Linear Algebra › Eigenvalues
Look for a linear relation among the columns before expanding. Here the first and third columns add up to the second because $sin^{2} θ + cos^{2} θ = 1$ and $- 3 + 7 = 4$ , so the determinant is zero.
Full solution
Q.571 markOfficial answer: C
Tap to open full size
Linear Algebra › Eigenvalues
Expand the characteristic determinant along the row with a single non-zero entry. After that the determinant factors into three linear terms, and the roots are the eigenvalues.
Full solution
Q.581 markOfficial answer: D
Tap to open full size
Earlier, ISRO's key gave A.
Engineering Mathematics › Calculus
In $\int\int\int f d x d y d z$ the innermost integral sign belongs to $d x$ and the outermost to $d z$ . Integrate from the inside out. Over $[- 3, 3]$ in $x$ the odd term $2 x y$ vanishes and the constant $- z^{3}$ is multiplied by $6$ . What is left is $\int_{1}^{2} - 6 z^{3} d z = - 22.5$ .
Full solution
Q.591 markOfficial answer: C
Tap to open full size
Artificial Intelligence › Logic
Rewrite the implication as a disjunction, $a \to b \equiv \neg a \lor b$ , then push the negation through the conjunction with De Morgan's law.
Full solution
Q.601 markOfficial answer: B
Tap to open full size
Probability and Statistics › Counting & axioms
Condition on whether the largest element is chosen. This gives $a_{n} = a_{n - 1} + a_{n - 2}$ , so the count of subsets of ${1, \dots, n}$ with no two consecutive elements is the Fibonacci number $F_{n + 2}$ .
Full solution
Q.611 markOfficial answer: D
Tap to open full size
Probability and Statistics › Counting & axioms
$A^{n}$ is the set of ordered $n$ -tuples over $A$ . Each of the $n$ positions has $m$ independent choices, so the count is $m \times m \times \dots \times m = m^{n}$ .
Full solution
Q.621 markOfficial answer: D
Tap to open full size
Probability and Statistics › Counting & axioms
With no repetition allowed, only the distinct letters matter. GAGANYAAN has just 4 distinct letters (G, A, N, Y), so each word uses all four once, giving $4!$ arrangements.
Full solution
Q.631 markOfficial answer: C
Tap to open full size
Operating Systems › Synchronisation
When the result of concurrent access to shared data depends on the order in which the accesses interleave, the situation is a race condition. Mutual exclusion around the critical section prevents it.
Full solution
Q.641 markOfficial answer: A
Tap to open full size
Earlier, ISRO's key gave D.
Operating Systems › Processes & threads
Creating a process (fork()) returns control to the parent at once, so the parent and the new child run concurrently. The parent blocks only if it later makes a separate wait()/waitpid() call. Waiting is an optional extra step, not part of process creation.
The question and its official answer
Q.651 markOfficial answer: D
Tap to open full size
Operating Systems › Processes & threads
A process that has finished waiting always returns to the Ready queue first. Only the scheduler's dispatch moves a process from Ready to Run.
Full solution
Q.661 markOfficial answer: A
Tap to open full size
Operating Systems › Deadlock
To convert a resource allocation graph (single-instance resources) to a wait-for graph, replace each path $P_{i} \to R \to P_{j}$ (request, then holder) by one edge $P_{i} \to P_{j}$ , and drop the resource nodes.
Full solution
Q.671 markOfficial answer: C
Tap to open full size
Operating Systems › CPU scheduling
FCFS orders processes by arrival time. Build the Gantt chart, then use turnaround $=$ completion $-$ arrival and waiting $=$ turnaround $-$ burst.
Full solution
Q.681 markOfficial answer: C
Tap to open full size
Operating Systems › CPU scheduling
Name each interval by its two end marks. Arrival to first CPU is wait. First CPU to finish is run. Arrival to first output is response. Arrival to finish is turnaround.
Full solution
Q.691 markOfficial answer: C
Tap to open full size
Operating Systems › Deadlock
Compute Need = Max - Allocation, then repeatedly pick a process whose Need fits within Work, let it finish, and add its allocation back to Work. If all processes can finish, the state is safe.
Full solution
Q.701 markOfficial answer: D
Tap to open full size
Operating Systems › Virtual memory
On each fault with full frames, look ahead in the reference string and evict the resident page whose next use is farthest away, or that is never used again. Count a fault only when the requested page is not already in a frame.
Full solution
Q.711 markOfficial answer: A
Tap to open full size
Q.721 markOfficial answer: B or C
Tap to open full size
Earlier, ISRO's key gave C.
Q.731 markOfficial answer: C
Tap to open full size
Q.741 markOfficial answer: D
Tap to open full size
Q.751 markExcluded from evaluation by ISRO
Tap to open full size
Earlier, ISRO's keys gave D, then A or D.
Q.761 markOfficial answer: A
Tap to open full size
Q.771 markOfficial answer: D
Tap to open full size
Q.781 markOfficial answer: C
Tap to open full size
Theory of Computation › Finite automata
$(a^{*} b)^{*}$ is $ε$ plus all strings ending in $b$ , and $(b^{*} a)^{*}$ is $ε$ plus all strings ending in $a$ . Together they cover every string.
Full solution
Q.791 markOfficial answer: B
Tap to open full size
Theory of Computation › Finite automata
Merge states that are equivalent. Here every final state leads only to final states, so all of them accept every string and collapse into one. The minimal DFA is the start state plus that single accepting state.
Full solution
Q.801 markOfficial answer: A
Tap to open full size
Theory of Computation › CFG & PDA
Each use of $S \to a S b$ adds one $a$ and one matching $b$ , while $T$ contributes at least one unmatched $b$ at the end, so the number of $b$ 's always exceeds the number of $a$ 's.
Full solution

Part B: Aptitude Q.81 to Q.95

Part B is the same aptitude test as in the ISRO Scientist/Engineer Electronics 2025 paper, held the same day, with the same question numbers. Its solutions are listed under ISRO EC 2025.

Q.811 markOfficial answer: D
Tap to open full size
General Aptitude › Numerical
Count odd days (days modulo 7) from a known reference date: an ordinary year adds 1 odd day, a leap year adds 2, and a whole number of weeks adds 0.
Full solution
Q.821 markExcluded from evaluation by ISRO
Tap to open full size
Earlier, ISRO's key gave B.
Q.832 marksOfficial answer: B
Tap to open full size
Earlier, ISRO's key gave A.
General Aptitude › Reasoning
A floor number tells you the least number of floors its building has. A 7th floor means at least 7 floors, which is more than 3, so the rule applies and that building has a lift. A conditional "if more than 3, then lift" says nothing about buildings with 3 or fewer floors.
Full solution
Q.841 markOfficial answer: C
Tap to open full size
General Aptitude › Geometry
Volume scales as the cube of the linear scale factor: doubling every side multiplies the volume by $2^{3} = 8$ .
Full solution
Q.851 markOfficial answer: B
Tap to open full size
General Aptitude › Numerical
When only one component is added, the other component's quantity stays fixed. Use it as the anchor and set up the new ratio against it.
Full solution
Q.862 marksOfficial answer: B
Tap to open full size
General Aptitude › Statistics & probability
Discrete variables take countable values and have a probability mass function. Poisson, binomial and geometric are the standard discrete laws. Gaussian, Rayleigh, exponential and uniform are continuous.
Full solution
Q.871 markOfficial answer: D
Tap to open full size
Earlier, ISRO's key gave C.
General Aptitude › Numerical
Subtract the sum of the people in a given group from the total to isolate the one who is left out, then back-substitute into the other equations.
Full solution
Q.881 markOfficial answer: C
Tap to open full size
General Aptitude › Reasoning
When neither conclusion follows by itself but the two are contradictories (a particular affirmative and a universal negative on the same two terms), one of them must be true, so the answer is 'Either I or II follows'.
Full solution
Q.892 marksOfficial answer: D
Tap to open full size
General Aptitude › Reasoning
A conclusion follows only if the statements force it. Statements about women teachers say nothing positive about other athletes, so claims that other athletes can play are not forced.
Full solution
Q.901 markOfficial answer: C
Tap to open full size
General Aptitude › Reasoning
When a number series grows quickly, take successive differences, and if they are not constant take differences of the differences. Here the differences are $2^{k} + 1$ , so the next one is $17$ .
Full solution
Q.911 markOfficial answer: B
Tap to open full size
General Aptitude › Spatial
A mirror image does two things at once: it reverses the order of the characters and it flips every character left to right. Check the first and last characters of each option first, then check the glyph shapes.
Full solution
Q.922 marksOfficial answer: D
Tap to open full size
General Aptitude › Numerical
Each circle's total includes its overlap regions, so subtract the labelled overlaps to find the unlabelled "only" regions. Then add only the regions lying outside the excluded circle.
Full solution
Q.931 markOfficial answer: D
Tap to open full size
General Aptitude › Spatial
In a pattern grid, follow each symbol on its own. Here the circle shifts one square right and the star shifts one square up in every box along a row, and the same two shifts hold in all three rows.
Full solution
Q.941 markOfficial answer: C
Tap to open full size
Earlier, ISRO's key gave A.
General Aptitude › Spatial
Separate the two marks. The black square follows a chessboard parity rule, and the crossed square repeats between cells a knight's move apart.
Full solution
Q.952 marksOfficial answer: A
Tap to open full size
General Aptitude › Spatial
Track two things separately: how the shape grows (one square added along a fixed path) and how the colouring behaves (alternating along the path, flipping its starting colour each figure).
Full solution

WHERE THE MARKS CAME FROM

Marks by subject in ISRO Scientist/Engineer Computer Science 2025
Subject	Qs	Marks
General Aptitude	14	19
Operating Systems	8	8
Database Management and Warehousing	6	6
Digital Electronics	6	6
Theory of Computation	6	6
Computer Networks	5	5
Data Structures and Algorithms	4	4
Discrete Mathematics	3	3
Probability and Statistics	3	3
Artificial Intelligence	2	2
Compiler Design	2	2
Linear Algebra	2	2
Engineering Mathematics	1	1
Machine Learning	1	1
Programming in C	1	1
Other questions	31	31

We sorted the questions into subjects ourselves; the exam paper does not label them.

Every ISRO paper ISRO Scientist/Engineer: who can apply CSE/IT study material DS & AI study material