UPSC Prelims PYQs with explanations
Real questions from official papers, mapped to syllabus topics. Attempt each one; the answer and the reason appear instantly.
Biological Oxygen Demand (BOD) is a standard criterion for
Answer (c) & explanation
BOD is used to test pollution in water (c). Biochemical (Biological) Oxygen Demand is the amount of dissolved oxygen that microbes use to break down the organic matter in a water sample, usually measured over 5 days at 20°C. A high BOD means heavy organic pollution, as from sewage or dairy waste, which takes oxygen away from fish. Clean river water has a BOD under about 3 mg/L, which is the bathing standard. It has nothing to do with blood (a), forests (b), or high altitudes (d).
COD (Chemical Oxygen Demand) measures all oxidisable matter and is always higher than BOD. Notice how the CPCB uses BOD to list polluted river stretches. Low dissolved oxygen with high BOD leads to eutrophication and fish deaths.
Answer as per the official UPSC answer key.
In India, Judicial Review implies
Answer (a) & explanation
Judicial review is the courts' power to rule on whether laws and executive orders are constitutional (a). In India, the Supreme Court and High Courts can test laws and government action against the Constitution and strike down anything that conflicts with it, mainly through Articles 13, 32 and 226. Courts do not judge the wisdom of a law (b), only whether it is valid. They do not check Bills before assent (c); only the advisory opinion route under Article 143 comes close. Reviewing its own judgements (d) is review jurisdiction under Article 137, which is different.
In Minerva Mills (1980) and L. Chandra Kumar (1997), the Supreme Court made judicial review part of the basic structure. Notice the US origin of the idea: Marbury v. Madison (1803).
(d) is "review" in the sense of Article 137, a different power with the same word.
Fell for this one? Most aspirants do. The PYQ Book puts every UPSC trap side by side, so the pattern finally clicks. Try the free sample
Answer as per the official UPSC answer key.
Consider the following :
A + B means A is the son of B.
A − B means A is the wife of B.
What does the expression P + R − Q mean?
Answer (c) & explanation
Q is P's father (c). Step 1: "A + B" means A is the son of B. So P + R means P is the son of R. Step 2: "A − B" means A is the wife of B. So R − Q means R is the wife of Q. Step 3: R is P's mother, and Q is R's husband. So Q is P's father. Option (a), Q is P's son, reverses the relation. Option (b), Q is P's wife, is wrong because Q is R's husband. Option (d) fails because (c) is clear.
Tip: decode coded relations one pair at a time, left to right. Draw a small family tree as you go; it makes the final link easy to read.
Answer as per the official UPSC answer key.
How many numbers are there between 99 and 1000 such that the digit 8 occupies the units place?
Answer (c) & explanation
There are 90 such numbers (c). Step 1: numbers between 99 and 1000 are the three-digit numbers, 100 to 999. Step 2: the units digit is fixed as 8. The hundreds digit can be 1 to 9, which is 9 ways. The tens digit can be 0 to 9, which is 10 ways. Step 3: total = 9 × 10 = 90. These are 108, 118, … up to 998. Option (b), 80, wrongly leaves out tens digit 0 or 8. Option (a), 64, and option (d), 104, do not fit.
Tip: in digit-counting questions, fill each place separately and multiply the choices. Remember the first digit cannot be 0, but other places can.
Answer as per the official UPSC answer key.
From the ecological point of view, which one of the following assumes importance in being a good link between the Eastern Ghats and the Western Ghats ?
Answer (a) & explanation
Sathyamangalam links the two Ghats (a). Sathyamangalam Tiger Reserve in Erode district, Tamil Nadu, sits where the Western and Eastern Ghats meet. It joins the Nilgiri Biosphere landscape, including Mudumalai, Bandipur and BRT Hills (Biligiri Rangaswamy Temple), giving tigers and elephants a vital corridor. It was declared a tiger reserve in 2013. The other options are wrong because Nallamala (b), home of the Nagarjunasagar-Srisailam reserve, and Seshachalam (d) lie entirely in the Eastern Ghats, while Nagarhole (c) is entirely in the Western Ghats.
BRT Hills is the other well-known meeting point of the two ranges. Notice the Moyar river gorge, which separates the Nilgiris from the Sathyamangalam forests.
Answer as per the official UPSC answer key.
Which of the following statements is/are true of the Fundamental Duties of an Indian citizen?
- 1. A legislative process has been provided to enforce these duties.
- 2. They are correlative to legal duties.
Select the correct answer using the code given below :
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong because Article 51A (Part IVA), added by the 42nd Amendment (1976) on the advice of the Swaran Singh Committee, lists duties but provides no way to enforce them. Statement 2 is wrong because the duties are not tied to legal duties in the Constitution. They are mainly moral obligations. Some laws do cover similar ground, such as the Prevention of Insults to National Honour Act, 1971 and the Wildlife and Forest Acts, but that is separate legislation.
There were 10 duties at first, and the 86th Amendment (2002) added an eleventh, requiring parents to educate children aged 6–14. Notice that the Verma Committee (1999) found ways to put some duties into practice. Courts also use them to judge whether laws are reasonable.
Answer as per the official UPSC answer key.
Which of the following practices can help in water conservation in agriculture ?
- 1. Reduced or zero tillage of the land
- 2. Applying gypsum before irrigating the field
- 3. Allowing crop residue to remain in the field
Select the correct answer using the code given below :
Answer (c) & explanation
Practices 1 and 3 save water (c). Reduced or zero tillage (1) leaves the soil structure intact, so less moisture is lost and more rain soaks in. Leaving crop residue on the field (3) works as a mulch that shades the soil, cuts evaporation and adds organic matter. The Happy Seeder machine sows wheat straight into rice stubble, which also cuts down stubble burning. Statement 2 is wrong because gypsum (calcium sulphate) is used to reclaim sodic (alkaline) soils and supply calcium and sulphur. It is not a water-saving practice.
Both 1 and 3 are parts of conservation agriculture, along with crop rotation. Notice the related water-saving methods: direct-seeded rice (DSR), alternate wetting and drying, and laser land levelling.
Answer as per the official UPSC answer key.
Which one of the following objectives is not embodied in the Preamble to the Constitution of India?
Answer (b) & explanation
Economic liberty is not in the Preamble (b). The Preamble promises liberty of thought, expression, belief, faith and worship, so options (a), (c) and (d) all appear in it. The economic idea in the Preamble is "Justice, social, economic and political", borrowed from the Russian Revolution. It does not mention economic liberty. It also promises equality of status and of opportunity and fraternity, ensuring the dignity of the individual and the unity and integrity of the nation.
The 42nd Amendment (1976) added "socialist, secular" and "integrity". The Supreme Court upheld these words in 2024. Notice that the ideals of liberty, equality and fraternity come from the French Revolution.
"economic" is in the Preamble, so (b) feels safe. It sits with justice, not liberty.
Answer as per the official UPSC answer key.
P = (40% of A) + (65% of B) and Q = (50% of A) + (50% of B), where A is greater than B. In this context, which of the following statements is correct?
Answer (d) & explanation
Nothing can be concluded with certainty (d). Step 1: P − Q = (0.40A + 0.65B) − (0.50A + 0.50B) = 0.15B − 0.10A. Step 2: the sign depends on how much bigger A is than B. If A = 1.2B, then 0.15B − 0.12B is positive, so P > Q. Step 3: if A = 2B, then 0.15B − 0.20B is negative, so P < Q. If A = 1.5B, P = Q. So options (a), (b) and (c) are each possible but not certain.
Tip: when an answer depends on values you are not given, test two very different cases. If they give different results, choose "cannot be concluded".
Answer as per the official UPSC answer key.
Read the passage
Productive jobs are vital for growth and a good job is the best form of inclusion. More than half of our population depends on agriculture, but the experience of other countries suggests that the number of people dependent on agriculture will have to shrink if per capita incomes in agriculture are to go up substantially. While industry is creating jobs, too many such jobs are low-productivity non-contractual jobs in the unorganized sector, offering low incomes, little protection, and no benefits. Service jobs are relatively of high productivity, but employment growth in services has been slow in recent years.
Which among the following is the most logical and rational inference that can be made from the above passage?
Answer (c) & explanation
We must create productive jobs outside agriculture while also raising farm productivity (c). Step 1: the passage says fewer people must depend on farming for farm incomes to rise. Industry and services both need productive jobs. So we need jobs outside farming and better farming. That is option (c). Step 2: option (a) covers services only, and leaves out industry. Step 3: option (b) says simply shift farm workers, ignoring farm productivity. Option (d) brings in hybrid and GM crops, which the passage does not discuss.
Tip: in India, close to half of workers are in agriculture, but it produces under a fifth of GDP. That is why moving workers to productive jobs matters.
Answer as per the official UPSC answer key.
The Trade Disputes Act of 1929 provided for
Answer (d) & explanation
The 1929 Act brought tribunals and a ban on some strikes (d). The Trade Disputes Act, 1929 set up courts of inquiry and boards of conciliation for industrial disputes. It banned strikes in public utility services without notice, and any strike meant to pressure the government. It did not give workers a role in management (a), it did not give management arbitrary powers (b), and it did not provide for British courts to intervene (c).
It came with the Public Safety Ordinance and the Meerut Conspiracy Case (1929), as the government cracked down on a labour movement growing under communist influence after the 1928 strikes in Bombay. Notice the earlier milestone: the Trade Unions Act, 1926 gave unions legal recognition.
Answer as per the official UPSC answer key.
If you travel by road from Kohima to Kottayam, what is the minimum number of States within India through which you can travel, including the origin and the destination ?
Answer (b) & explanation
The minimum is seven States (b). The shortest route by number of States is Nagaland → Assam → West Bengal → Odisha → Andhra Pradesh → Tamil Nadu → Kerala, which is seven States, including the start and end. The narrow Siliguri Corridor ("Chicken's Neck") makes passing through West Bengal unavoidable. Going inland through Bihar, Jharkhand, Chhattisgarh or Telangana adds States, so 8 (c) or 9 (d) is not the minimum, and 6 (a) is not possible.
The national highway along the east coast makes this route practical. Notice the strategic point: the Siliguri Corridor, about 20–22 km wide, is the northeast's only land link to the rest of India, which is why projects through Bangladesh and the Kaladan route matter.
Answer as per the official UPSC answer key.
'Broad-based Trade and Investment Agreement (BTIA)' is sometimes seen in the news in the context of negotiations held between India and
Answer (a) & explanation
The BTIA was negotiated between India and the European Union (a). The Broad-based Trade and Investment Agreement was the name for the India–EU free trade talks that began in 2007. They stalled in 2013 over car and wine tariffs, data security and the movement of professionals. They were relaunched in 2022 as three tracks: a trade agreement, an investment protection agreement, and a geographical indications pact. India and the EU concluded the FTA on 27 January 2026 at the 16th India–EU Summit. The GCC (b), OECD (c) and SCO (d) are not linked to the BTIA.
The FTA still needs signing and ratification, and is expected to take effect around 2027. Notice the sticking point that remains: the EU's CBAM carbon border tax.
Answer as per the official UPSC answer key.
Consider the given information and answer the item that follows.
No supporters of 'party X', who knew Z and supported his campaign strategy, agreed for the alliance with 'party Y'; but some of them had friends in 'party Y'.
With reference to the above information, which one among the following statements must be true?
Answer (b) & explanation
Option (b) must be true: at least one supporter of party Y knows some supporter of party X as a friend (b). Step 1: the information says some X supporters, who knew Z and backed his strategy, "had friends in party Y". Step 2: a friendship goes both ways. So at least one Y supporter has an X supporter as a friend. That is option (b). Step 3: options (c) and (d) say no X supporter backed Z's strategy or knew Z. Both contradict the information. Option (a) is about what Y supporters thought of the alliance, which is never given.
Tip: "must be true" needs proof from the facts. Turn the given sentence around; friendship is mutual, so the reverse also holds.
Answer as per the official UPSC answer key.
Read the passage
Disruption of traditional institutions, identifications and loyalties is likely to lead to ambivalent situations. It is possible that some people may renew their identification with traditional groups whereas others align themselves with new groups and symbols emergent from processes of political development. In addition, political development tends to foster group awareness of a variety of class, tribe, region, clan, language, religion, occupation and others.
Which one of the following is the best explanation of the above passage?
Answer (a) & explanation
The best explanation is that political development is not a one-way process; it involves both growth and decay (a). Step 1: the passage says some people renew old loyalties while others join new groups. So change moves in more than one direction. That is option (a). Step 2: option (c) says breaking away from old loyalties is impossible. But some people do join new groups. Step 3: option (b), that traditional societies succeed in resisting good change, is not claimed. Option (d) says old loyalties help development, which the passage does not say.
Tip: "unilinear" means moving in a single straight line. When a passage shows movement both ways, pick the option that says change is mixed.
Answer as per the official UPSC answer key.
A 2-digit number is reversed. The larger of the two numbers is divided by the smaller one. What is the largest possible remainder?
Answer (d) & explanation
The largest possible remainder is 45 (d). Step 1: when the quotient is 1, the remainder is just the difference of the two numbers. That difference is always 9 × (difference of the digits). Step 2: to make the difference large while the quotient stays 1, the bigger number must be less than twice the smaller. Take digits 9 and 4: 94 ÷ 49 = 1, remainder 45. Step 3: try 95 and 59: 95 ÷ 59 = 1, remainder 36. Bigger digit gaps make the quotient 2 or more and the remainder smaller. Options (a), (b) and (c) are possible remainders but not the largest.
Tip: test the highest option first. If it works and nothing beats it, you are done.
Answer as per the official UPSC answer key.
The object of the Butler Committee of 1927 was to
Answer (d) & explanation
The Butler Committee was about relations between the Government of India and the princely states (d). The Indian States Committee under Sir Harcourt Butler (1927–29) looked into ties between the paramount power and the princely states. The princes hoped it would limit British interference. Instead it said paramountcy must remain supreme and could change with the times. It also said a state could not be handed to an elected Indian government without its own consent. It did not define Centre–province powers (a), which belonged to the Simon Commission's work. It did not deal with the Secretary of State (b) or press censorship (c).
The Chamber of Princes (1921) was the princes' forum. Notice how this consent principle later helped the princes resist the 1935 federation plan.
Answer as per the official UPSC answer key.
With reference to the role of UN-Habitat in the United Nations programme working towards a better urban future, which of the statements is/are correct ?
- 1. UN-Habitat has been mandated by the United Nations General Assembly to promote socially and environmentally sustainable towns and cities to provide adequate shelter for all.
- 2. Its partners are either governments or local urban authorities only.
- 3. UN-Habitat contributes to the overall objective of the United Nations system to reduce poverty and to promote access to safe drinking water and basic sanitation.
Select the correct answer using the code given below :
Answer (b) & explanation
Statements 1 and 3 are correct (b). UN-Habitat, based in Nairobi (1978), has a UN General Assembly mandate to promote socially and environmentally sustainable towns and cities and adequate shelter for all (1). It also supports the UN's wider aims of cutting poverty and improving access to safe water and sanitation (3), and it leads on SDG 11 (sustainable cities). Statement 2 is wrong because its partners are not only governments and local authorities. They also include NGOs, academia, the private sector and community groups.
It holds the World Urban Forum and produces the World Cities Report. Notice India's urban missions: PMAY-Urban 2.0, AMRUT 2.0 and Swachh Bharat (Urban) 2.0.
Answer as per the official UPSC answer key.
With reference to Indian freedom struggle, consider the following events :
- 1. Mutiny in Royal Indian Navy
- 2. Quit India Movement launched
- 3. Second Round Table Conference
What is the correct chronological sequence of the above events ?
Answer (c) & explanation
The order is 3-2-1 (c). The Second Round Table Conference sat in London from September to December 1931. Gandhi attended as Congress's only representative after the Gandhi–Irwin Pact, and he came back empty-handed. Quit India was launched on 8 August 1942 at Gowalia Tank, Bombay, with the call "Do or Die". The Royal Indian Navy mutiny broke out at Bombay on 18 February 1946, starting on HMIS Talwar. Any other order (a, b, d) is wrong.
The RIN mutiny, along with the INA trials (Red Fort, 1945), convinced Britain it could no longer count on Indian forces. Notice what came next: the Cabinet Mission arrived in March 1946.
Answer as per the official UPSC answer key.
Gopal bought a cell phone and sold it to Ram at 10% profit. Then Ram wanted to sell it back to Gopal at 10% loss. What will be Gopal's position if he agreed?
Answer (c) & explanation
Gopal gains 1% (c). Step 1: suppose Gopal first paid Rs 100 for the phone. He sold it to Ram at 10% profit, for Rs 110. Step 2: Ram sells it back at 10% loss on his own price. 10% of 110 = 11, so the price is Rs 99. Step 3: Gopal now has the same phone for Rs 99 instead of Rs 100. That is Rs 1 less on every Rs 100 he first spent. That is a gain of 1%. Option (a), no loss no gain, is the trap of thinking 10% up and 10% down cancel. Options (b) and (d) do not fit.
Tip: percentages up and down use different bases, so they never simply cancel.
Counting all the cash (+110 − 100 − 99), some say Gopal is better off by ₹ 11; the official answer is (c), as printed.
Answer as per the official UPSC answer key.