Science and Technology: previous-year questions
185 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 50 of them came in the last five papers.
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Which one of the following is the context in which the term “qubit” is mentioned ?
Answer (b) & explanation
A qubit relates to quantum computing (b). A qubit, or quantum bit, is the basic unit of quantum computing. An ordinary bit is either 0 or 1. Through superposition, a qubit can be 0 and 1 at once, and qubits can be entangled, so their states are linked. This lets quantum computers solve some problems, such as cryptography and molecular modelling, far faster. It is not a term from cloud services (a), visible light communication (c) such as Li-Fi, or wireless technology (d).
Notice India's National Quantum Mission (2023), with about ₹6,000 crore, which aims to build 50–1,000 qubit machines within eight years. 2025 was the UN's International Year of Quantum Science and Technology.
Answer as per the official UPSC answer key.
Consider the following communication technologies :
- 1. Closed-circuit Television
- 2. Radio Frequency Identification
- 3. Wireless Local Area Network
Which of the above are considered Short-Range devices/technologies ?
Answer (d) & explanation
All three are short-range technologies (d). Short-Range Devices (SRDs) are low-power radio transmitters that work over short distances and have little potential to interfere with other services. Item 1 is right: wireless CCTV links send video over short ranges. Item 2 is right: RFID tags, used in FASTag and shop security, are read from close range. Item 3 is right: Wireless LAN, or Wi-Fi, covers a home or office. Bluetooth, NFC, wireless alarms and car remotes are other SRDs.
Notice that the Department of Telecommunications delicenses many SRD bands, such as parts of the 2.4 GHz and 5 GHz bands for Wi-Fi, so no licence is needed. In 2024, it delicensed the lower 6 GHz band for Wi-Fi.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Biofilms can form on medical implants within human tissues.
- 2. Biofilms can form on food and food processing surfaces.
- 3. Biofilms can exhibit antibiotic resistance.
Which of the statements given above are correct ?
Answer (d) & explanation
All three statements are correct (d). A biofilm is a colony of microbes stuck to a surface inside a slimy matrix they make themselves. Statement 1 is right: biofilms form on catheters, artificial joints and other medical implants, causing hard-to-treat infections. Statement 2 is right: they form on food and food-processing surfaces, spreading germs such as Listeria. Statement 3 is right: the matrix shields the bacteria, making them far more resistant to antibiotics, sometimes 1,000 times more.
Notice the everyday example: dental plaque is a biofilm. Biofilms also clog pipes and harm ship hulls. They add to the problem of antimicrobial resistance, which the WHO lists among the top global health threats.
Answer as per the official UPSC answer key.
Consider the following statements in respect of probiotics :
- 1. Probiotics are made of both bacteria and yeast.
- 2. The organisms in probiotics are found in foods we ingest but they do not naturally occur in our gut.
- 3. Probiotics help in the digestion of milk sugars.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). Probiotics are live, helpful microbes taken in food or supplements. Statement 1 is right: most are bacteria, such as Lactobacillus and Bifidobacterium, and some are yeasts, such as Saccharomyces boulardii. Statement 2 is wrong: the same kinds of microbes live naturally in our gut, as part of the gut microbiome. Statement 3 is right: lactobacilli make the enzyme lactase, which helps digest lactose, the milk sugar, easing lactose intolerance.
Notice the related terms: prebiotics are fibres that feed good bacteria, and synbiotics combine both. Curd (dahi), yoghurt and kefir are common probiotic foods. Probiotics are regulated in India by FSSAI.
Answer as per the official UPSC answer key.
In the context of vaccines manufactured to prevent COVID-19 pandemic, consider the following statements :
- 1. The Serum Institute of India produced COVID-19 vaccine named Covishield using mRNA platform.
- 2. Sputnik V vaccine is manufactured using vector based platform.
- 3. COVAXIN is an inactivated pathogen based vaccine.
Which of the statements given above are correct ?
Answer (b) & explanation
Statements 2 and 3 are correct (b). Statement 1 is wrong: Covishield, the Oxford–AstraZeneca vaccine made by the Serum Institute of India, uses a chimpanzee adenovirus vector (ChAdOx1), not mRNA. Statement 2 is right: Sputnik V, from Russia's Gamaleya Institute, uses two human adenovirus vectors (Ad26 and Ad5). Statement 3 is right: Bharat Biotech's Covaxin uses a whole inactivated virus that cannot cause disease.
Notice that the mRNA vaccines were Pfizer-BioNTech's, Moderna's and India's own Gemcovac (Gennova). The 2023 Nobel Prize in Medicine went to Katalin Karikó and Drew Weissman for the mRNA work. Covishield was withdrawn in 2024 after rare clotting side effects drew attention.
Answer as per the official UPSC answer key.
If a major solar storm (solar flare) reaches the Earth, which of the following are the possible effects on the Earth ?
- 1. GPS and navigation systems could fail.
- 2. Tsunamis could occur at equatorial regions.
- 3. Power grids could be damaged.
- 4. Intense auroras could occur over much of the Earth.
- 5. Forest fires could take place over much of the planet.
- 6. Orbits of the satellites could be disturbed.
- 7. Shortwave radio communication of the aircraft flying over polar regions could be interrupted.
Select the correct answer using the code given below :
Answer (c) & explanation
Effects 1, 3, 4, 6 and 7 are possible (c). A big solar storm disturbs Earth's ionosphere and magnetic field. Item 1 is right: it disrupts GPS and navigation signals. Item 3 is right: it induces currents that can damage power grids, as in Quebec in 1989. Item 4 is right: it spreads auroras to low latitudes. Item 6 is right: it heats the upper air, which drags satellites down; SpaceX lost about 40 Starlink satellites in 2022. Item 7 is right: it blocks shortwave (HF) radio over the poles. Items 2 and 5 are wrong: it cannot cause tsunamis or forest fires.
Notice the Carrington Event (1859), the strongest storm on record. India's Aditya-L1 (2023) studies the Sun from the L1 point.
Remove the two impossible effects, tsunamis and forest fires. Only option (c) is left.
Answer as per the official UPSC answer key.
Which one of the following statements best describes the role of B cells and T cells in the human body ?
Answer (d) & explanation
B cells and T cells protect the body from pathogens (d). B and T lymphocytes are white blood cells that carry out adaptive immunity, which targets particular germs. B cells make antibodies that tag germs. Cytotoxic T cells kill infected cells, and helper T cells organise the whole response. Both keep a memory of past germs, which is why vaccines work. Option (a) is wrong: allergies come from an overreaction of the immune system, not protection from it. Option (b) is wrong: pain and inflammation are eased by drugs like painkillers. Option (c) is wrong: these cells drive immunity; they do not suppress it.
Notice that HIV attacks helper T cells (CD4). CAR-T therapy, now made in India, reprograms a patient's T cells to fight cancer.
Answer as per the official UPSC answer key.
With reference to Ayushman Bharat Digital Mission, consider the following statements :
- 1. Private and public hospitals must adopt it.
- 2. As it aims to achieve universal health coverage, every citizen of India should be part of it ultimately.
- 3. It has seamless portability across the country.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Only statement 3 is correct (b). The Ayushman Bharat Digital Mission (2021) builds digital health infrastructure: the ABHA (Ayushman Bharat Health Account) health ID, and registries of health professionals and facilities. Statement 1 is wrong: joining is voluntary for both private and public hospitals. Statement 2 is wrong: it is also voluntary for citizens; no one must take an ABHA ID. Statement 3 is right: records linked to ABHA are portable across hospitals, labs and States, with the patient's consent.
Notice that ABDM is run by the National Health Authority, which also runs AB-PMJAY, the health insurance scheme of ₹5 lakh a family. In 2024, PMJAY was extended to all citizens aged 70 and above.
"Must" and "every citizen" are the words that rule out statements 1 and 2.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Aarogya Setu
- 2. CoWIN
- 3. DigiLocker
- 4. DIKSHA
Which of the above are built on top of open-source digital platforms ?
Answer (d) & explanation
All four are built on open-source platforms (d). All four are part of India's digital public infrastructure, built on open-source code and open standards. Item 1 is right: Aarogya Setu, the COVID contact-tracing app, made its code open source in 2020. Item 2 is right: CoWIN, the vaccine registration platform, became an open-source stack (DIVOC) offered to other countries. Item 3 is right: DigiLocker stores official documents digitally on open standards. Item 4 is right: DIKSHA, the school learning platform, is built on Sunbird, an open-source framework.
Notice the India Stack: Aadhaar, UPI, DigiLocker and consent-based data sharing. India pushed DPI globally during its G20 presidency (2023), with the Global DPI Repository.
Answer as per the official UPSC answer key.
With reference to Web 3.0, consider the following statements :
- 1. Web 3.0 technology enables people to control their own data.
- 2. In Web 3.0 world, there can be blockchain based social networks.
- 3. Web 3.0 is operated by users collectively rather than a corporation.
Which of the statements given above are correct ?
Answer (d) & explanation
All three statements are correct (d). Web 3.0 is the idea of a decentralised internet built on blockchains. Statement 1 is right: users hold their own data and digital assets in wallets, rather than leaving them with big platforms. Statement 2 is right: applications, including social networks, can run on public blockchains. Statement 3 is right: decisions can be made by users collectively, for example through DAOs (decentralised autonomous organisations), rather than by one company.
Notice the earlier stages: Web 1.0 was read-only static pages; Web 2.0 is today's interactive, platform-run web of social media. Web 3.0 is linked with cryptocurrencies and NFTs. India taxes crypto gains at 30%, with 1% TDS.
Answer as per the official UPSC answer key.
Bollgard I and Bollgard II technologies are mentioned in the context of
Answer (b) & explanation
Bollgard refers to genetically modified crops (b). Bollgard is Monsanto's (now Bayer's) Bt cotton. Bollgard I carries the cry1Ac gene from the soil bacterium Bacillus thuringiensis, and Bollgard II adds cry2Ab. These genes make toxins that kill the bollworm, a major cotton pest. It is not about clonal propagation (a), growth substances (c) or biofertilisers (d).
Notice that Bt cotton, approved in 2002, is India's only commercially approved GM crop, though GM mustard (DMH-11) got environmental release approval in 2022. Pink bollworm has developed resistance, weakening Bt cotton. The GEAC under the Environment Ministry approves GM crops. India released two genome-edited rice varieties in 2025.
Answer as per the official UPSC answer key.
In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following?
- 1. Area of the hole in the lid
- 2. Temperature of the flame
- 3. Weight of the lid
Select the correct answer using the code given below.
Answer (c) & explanation
The hole area and lid weight set the temperature (c). Water boils at a higher temperature when the pressure is higher. In a pressure cooker, the pressure inside is set by the weight of the whistle on the vent divided by the area of the hole. Item 3 is right: a heavier weight holds more pressure. Item 1 is right: a smaller hole means the same weight holds more pressure. Item 2 is wrong: a hotter flame only makes the cooker reach that pressure sooner and release more steam; it does not raise the cooking temperature.
Notice the reverse case: at high altitude, low air pressure makes water boil below 100 °C, so food cooks slowly. That is why pressure cookers are vital in the mountains. Typical cookers reach about 120 °C.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Bacteria
- 2. Fungi
- 3. Virus
Which of the above can be cultured in artificial/synthetic medium?
Answer (a) & explanation
Bacteria and fungi can be grown on artificial media (a). Item 1 is right: bacteria grow on nutrient agar or in broth, which supply sugars, salts and proteins. Item 2 is right: fungi also grow on artificial media, such as Sabouraud agar. Item 3 is wrong: viruses cannot reproduce on their own. They are not cells and have no machinery to make proteins, so they need living cells, such as cell cultures, embryonated eggs or animals. They cannot be grown on a cell-free synthetic medium.
Notice that most flu vaccines are still made by growing the virus in hens' eggs. The difference matters in medicine: antibiotics work against bacteria, not viruses. Bacteriophages are viruses that infect bacteria.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Adenoviruses have single-stranded DNA genomes whereas retroviruses have double-stranded DNA genomes.
- 2. Common cold is sometime caused by an adenovirus whereas AIDS is caused by a retrovirus.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). Statement 1 is wrong on both counts. Adenoviruses have double-stranded DNA genomes. Retroviruses, such as HIV, carry single-stranded RNA, which an enzyme called reverse transcriptase copies into DNA that joins the host's genome. Statement 2 is right: adenoviruses cause some common colds, sore throats and conjunctivitis, while HIV, a retrovirus, causes AIDS.
Notice that most colds are caused by rhinoviruses, with adenoviruses and coronaviruses behind some. Adenoviruses are also used as vectors in vaccines such as Covishield. HIV treatment uses antiretroviral therapy (ART), and India's National AIDS Control Programme began in 1992.
Answer as per the official UPSC answer key.
Water can dissolve more substances than any other liquid because
Answer (a) & explanation
Water dissolves so much because it is dipolar (a). A water molecule is polar, or dipolar: the oxygen end carries a slight negative charge, and the two hydrogen ends carry slight positive charges. These partial charges surround ions and other polar molecules and pull them apart, so water dissolves more substances than any other liquid. That is why it is called the "universal solvent". Options (b) and (c) are true of water but do not explain dissolving. Option (d) is true, but being an oxide of hydrogen is not the reason.
Notice water's other special traits: high specific heat, which moderates climate; hydrogen bonding; and ice being less dense than liquid water, so it floats.
Answer as per the official UPSC answer key.
With reference to street-lighting, how do sodium lamps differ from LED lamps?
- 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
- 2. As street-lights, sodium lamps have longer life span than LED lamps.
- 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.
Select the correct answer using the code given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Statement 1 is right: a sodium lamp glows in all directions (360°), so much light is wasted unless reflectors are used, while LEDs are directional. Statement 3 is right: sodium lamps give a nearly monochromatic yellow light, so colours are hard to tell apart, while LEDs render colours far better. Statement 2 is wrong: LEDs last longer than sodium lamps, often 50,000 hours or more, and use much less power.
Notice India's programmes: UJALA (2015) for LED bulbs in homes, and the Street Lighting National Programme (SLNP), run by EESL, which replaced over 1.3 crore street lights with LEDs. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
The term ‘ACE2’ is talked about in the context of
Answer (d) & explanation
ACE2 relates to the spread of viral diseases (d). ACE2 (angiotensin-converting enzyme 2) is a protein on the surface of cells in the lungs, gut, heart and blood vessels. The spike protein of SARS-CoV-2 binds to ACE2 to enter cells, like a key in a lock. That is why it became central to the study of how COVID-19 spreads and harms the body. It is not a gene in GM plants (a), part of NavIC (b) or radio collars (c).
Notice that the earlier SARS virus (2002–03) also used ACE2. Normally, ACE2 helps control blood pressure. Many vaccines target the spike protein to stop it binding to ACE2.
Answer as per the official UPSC answer key.
Which one of the following is a reason why astronomical distances are measured in light-years?
Answer (d) & explanation
Light-years work because the speed of light is constant (d). A light-year is the distance light covers in one year in a vacuum, about 9.46 × 10¹² km. It can serve as a unit only because the speed of light in a vacuum is always the same, about 3 × 10⁵ km per second, for every observer. Option (a) is wrong: distances between stars and galaxies do change; the universe is even expanding. Option (b) is wrong: gravity of stellar bodies changes as they evolve. Option (c) is wrong: light does not always travel straight; gravity bends it (gravitational lensing).
Notice the other units: the astronomical unit (Earth–Sun distance, about 150 million km) and the parsec (about 3.26 light-years). The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away.
Answer as per the official UPSC answer key.
With reference to recent developments regarding ‘Recombinant Vector Vaccines’, consider the following statements :
- 1. Genetic engineering is applied in the development of these vaccines.
- 2. Bacteria and viruses are used as vectors.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). A recombinant vector vaccine puts a gene for a pathogen's antigen into a harmless carrier, which then makes the antigen inside the body and trains the immune system. Statement 1 is right: making the vaccine relies on genetic engineering. Statement 2 is right: the carrier can be a virus, such as the adenovirus in Covishield and Sputnik V, or a weakened bacterium, such as Salmonella or BCG.
Notice other COVID vaccine types: Covaxin used a whole inactivated virus; Pfizer and Moderna used mRNA; ZyCoV-D was the world's first DNA vaccine for people; Corbevax was a protein subunit vaccine. India also approved an intranasal vaccine (iNCOVACC).
Answer as per the official UPSC answer key.
In the context of hereditary diseases, consider the following statements :
- 1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg.
- 2. A child inherits mitochondrial diseases entirely from mother and not from father.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). Statement 2 is right: mitochondria come only from the egg, because the sperm's mitochondria are destroyed after fertilisation. So mitochondrial DNA diseases pass down entirely from the mother. Statement 1 is right: mitochondrial replacement therapy swaps the mother's faulty mitochondria for healthy donor mitochondria. This can be done before fertilisation (spindle transfer) or after it (pronuclear transfer), with in vitro fertilisation. The child then carries DNA from three people, hence "three-parent babies".
Notice that the UK legalised the method in 2015, and in 2025 reported eight healthy babies born this way. Mitochondria are the cell's powerhouses, making energy as ATP.
Answer as per the official UPSC answer key.