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.

The Red Data Book contains information about endangered and threatened species of plants and animals. It is published by the International Union for Conservation of Nature (IUCN). The book categorizes species based on their risk of extinction, such as Critically Endangered, Endangered, Vulnerable, and Extinct. It serves as a global reference for monitoring biodiversity conservation and guiding environmental protection policies.

Other options (for context)

• All plant species: Incorrect — it includes only those plants that are endangered or threatened.
• All animal species: Incorrect — it covers only the endangered and vulnerable ones, not all animals.
• All extinct species: Incorrect — it includes species at risk of extinction, not only those already extinct.

The gravitational force of the Earth is maximum at the poles and minimum at the equator. This happens because the Earth is not a perfect sphere; it is slightly flattened at the poles and bulged at the equator due to its rotation. The distance from the center of the Earth to the poles is slightly less than that to the equator, resulting in a stronger gravitational pull at the poles.

Other options (for context)

• At equator: Gravity is slightly weaker because of the Earth's bulge and centrifugal force due to rotation.
• At tropic of cancer: Gravity here is intermediate, not maximum.
• At tropic of Capricorn: Same as the Tropic of Cancer — gravity is moderate, not maximum.

Explanation: The HIV (Human Immunodeficiency Virus) primarily attacks the lymphocytes, especially the CD4⁺ T-helper cells of the immune system. This weakens the body’s defense mechanism, making it vulnerable to various infections and diseases, eventually leading to AIDS (Acquired Immunodeficiency Syndrome).

Explanation: CNG stands for Compressed Natural Gas. It is an eco-friendly fuel made primarily of methane (CH₄) and is used as an alternative to petrol and diesel. CNG burns cleanly, producing fewer pollutants and helping reduce air pollution.

Explanation: Bilirubin and Biliverdin are yellow and green bile pigments respectively, found in the bile secreted by the liver. These pigments are produced during the breakdown of hemoglobin from old red blood cells (RBCs) in the liver and spleen. Biliverdin (green pigment) is first formed from the heme part of hemoglobin and is then reduced to bilirubin (yellow pigment). The bile containing these pigments helps in the digestion and emulsification of fats. Excess bilirubin in the blood causes jaundice, leading to a yellowing of the skin and eyes.

In medical terminology, USG stands for Ultrasonography. It is a diagnostic imaging technique that uses high-frequency sound waves to create images of internal organs, tissues, and developing fetuses. USG is commonly used in obstetrics, cardiology, and abdominal examinations as it is safe and non-invasive.

• Undersized gland: Not a recognized medical term.
• Unsegmented gonads: Incorrect and unrelated to imaging.
• None of the above: Incorrect as option B is correct.

Explanation: ELISA stands for Enzyme-linked immunosorbent assay. It is a biochemical test used to detect and measure antibodies, antigens, proteins, or hormones in a sample. The method relies on an enzyme-linked antigen or antibody and a color change to indicate a positive reaction.