Explanation: A rainbow is formed in the part of the sky opposite to the Sun. It occurs when sunlight is refracted, internally reflected, and dispersed by raindrops, splitting the light into its seven constituent colors. The observer must have the Sun behind them and rain in front to see the rainbow clearly.

Explanation: Cow’s milk is a nutritionally rich food containing proteins, fats, carbohydrates, minerals, and vitamins. It is particularly rich in B-group vitamins, especially Vitamin B₂ (Riboflavin) and Vitamin B₁₂ (Cobalamin). These vitamins play crucial roles in maintaining healthy skin, eyesight, and the nervous system, as well as aiding in the formation of red blood cells.

Note: While milk provides moderate amounts of Vitamin A and D, the natural content of Vitamin D is low, and milk is often fortified with Vitamin D to enhance its nutritional value.

Explanation: Web pages are written using HTML (HyperText Markup Language). It is the standard language used to create and structure content on the web by defining elements such as headings, paragraphs, links, images, and tables.

In organic farming, chemical fertilizers are not used. Instead, farmers rely on natural manures, compost, and biofertilizers to enrich the soil. Organic farming avoids synthetic pesticides and focuses on maintaining ecological balance, soil fertility, and sustainable crop production.

• Intercropping: Often used to increase productivity naturally.
• Manures: Essential in organic farming for soil health.
• Biofertilizers: Widely used to promote natural nutrient availability.

Explanation: The permissible noise level in residential areas should not exceed 55 decibels (dB) during the daytime and 45 dB at night, as per the Central Pollution Control Board (CPCB) guidelines. Exposure to higher noise levels can lead to stress, disturbed sleep, and hearing impairment.

Explanation: PVC (Polyvinyl Chloride) is an example of a thermoplastic. Thermoplastics soften on heating and can be reshaped multiple times without undergoing any chemical change. In contrast, Bakelite and Melamine are thermosetting plastics that become hard and rigid after heating and cannot be remolded.

Explanation: The element hydrogen is common to all acids. Acids are substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. This property gives acids their characteristic sour taste, the ability to turn blue litmus paper red, and to react with bases to form salts and water. Examples include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃)—all of which release hydrogen ions in aqueous solution.

India’s first successful atomic device was tested at Pokhran in the Thar Desert of Rajasthan on 18th May 1974. The operation was code-named "Smiling Buddha" and was conducted by the Bhabha Atomic Research Centre (BARC) under the leadership of scientists like Dr. Raja Ramanna. This event marked India’s entry into the group of nuclear-capable nations.

Other options (for context)

• Bikaner: A city in Rajasthan, but no nuclear tests were conducted there.
• Barmer: Located near the Pakistan border, not the site of India’s nuclear test.
• Jaisalmer: Close to Pokhran geographically, but not the test location.

Explanation: The pairing Louis Pasteur : Blood groups is not correctly matched. Louis Pasteur is known for his work on pasteurization and germ theory of disease, not for discovering blood groups. The discovery of blood groups was made by Karl Landsteiner in 1900. Other matches are correct.