Explanation: Proteins are complex organic compounds composed of smaller units called amino acids. Each amino acid contains an amino group (–NH₂), which contributes nitrogen to the molecular structure. This makes nitrogen an essential element in all proteins. Carbohydrates and fats lack nitrogen, while only some vitamins contain it.

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.

Explanation: Lysergic acid diethylamide (LSD) was discovered by Dr. Albert Hofmann, a Swiss chemist, in 1938 at the Sandoz Laboratories in Switzerland. LSD is a powerful hallucinogenic compound derived from lysergic acid, which occurs in the fungus Claviceps purpurea (ergot) that grows on rye. Hofmann accidentally discovered its psychological effects in 1943 when he absorbed a small amount through his skin, leading to vivid visual hallucinations. LSD acts on serotonin receptors in the brain and profoundly alters perception and mood.

Explanation: The scientific name of Vitamin C is Ascorbic Acid. It is a water-soluble vitamin essential for the growth, repair, and maintenance of body tissues. Vitamin C acts as a powerful antioxidant and helps in the synthesis of collagen, absorption of iron, and boosting the immune system. A deficiency of Vitamin C leads to the disease Scurvy, characterized by bleeding gums, weakness, and delayed wound healing. Natural sources include citrus fruits like oranges, lemons, amla, and guava.

Explanation: The pair Faraday : Diffusion of gases is incorrect. The law of diffusion of gases was given by Thomas Graham, not Michael Faraday. Faraday is known for his work on electromagnetism and electrolysis. The other pairs are correctly matched.

Explanation: Mucor is not a biofertilizer; it is a saprophytic fungus that grows on decaying organic matter. Biofertilizers are living microorganisms that enhance soil fertility by fixing atmospheric nitrogen or solubilizing nutrients. Examples include Rhizobium, Azotobacter, and Anabaena.

Nickel exposure commonly causes a skin disease called Dermatitis, specifically Allergic Contact Dermatitis. This condition occurs when the skin comes into direct contact with nickel-containing objects such as jewelry, coins, or belt buckles. It leads to redness, itching, and rashes, especially in sensitive individuals. Nickel allergy is one of the most common metal allergies worldwide.

Other options (for context)

• Itai Itai: Caused by cadmium poisoning, not nickel.
• Learning Disability: Not directly linked to nickel exposure.
• Asthma: May be aggravated by pollutants, but not typically caused by nickel.

Explanation: Diamond is the hardest natural form of carbon. In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral structure, forming a strong three-dimensional lattice. This arrangement makes diamond extremely hard, dense, and transparent. It is used in cutting, drilling, and polishing tools. In contrast, graphite (another allotrope of carbon) is soft and slippery because its carbon atoms are arranged in layers with weak forces between them.

Explanation: Red light is used as a stop signal because it has the longest wavelength among visible colors, allowing it to scatter the least and remain visible from a greater distance, even in fog, rain, or dust. This ensures better visibility and safety on roads.

Explanation: A carnivore is an animal that primarily eats the flesh of other animals. The tiger (Panthera tigris) is a true carnivore and an apex predator in its ecosystem. It has sharp canines and retractable claws adapted for hunting and tearing flesh. Tigers play a vital role in maintaining the ecological balance by controlling herbivore populations in forests.