Explanation: The hard structure of bones and teeth is primarily made of calcium phosphate in the crystalline form of tricalcium phosphate (Ca₃(PO₄)₂). It provides rigidity and strength. In tooth enamel, a small portion is converted into fluorapatite, which makes teeth resistant to bacterial decay.

Explanation: The femur is the single bone of the thigh region and extends from the hip joint to the knee joint. It supports the body’s weight during standing, walking, and running. The femur is the longest, heaviest, and strongest bone in the human skeleton. It articulates with the pelvic girdle at its proximal end (head of femur fits into the acetabulum of the pelvis) and with the tibia and patella at the distal end.

Average Length: Approximately 45 cm (about 1/4 of body height).

The Stockholm Convention is not associated with the reduction of greenhouse gases. It focuses on eliminating or restricting the production and use of persistent organic pollutants (POPs) — toxic chemicals that remain in the environment for long periods, such as DDT and PCBs. It does not directly address greenhouse gas emissions or climate change.

Other options (for context)

• Kigali Amendment to the Montreal Protocol: Aims to phase down the use of hydrofluorocarbons (HFCs), which are potent greenhouse gases used in refrigerants — hence it helps reduce global warming.
• Kyoto Protocol: An international treaty adopted in 1997 to reduce greenhouse gas emissions by developed countries, directly targeting climate change.

Explanation: Measles is a highly contagious viral disease caused by the measles virus (a member of the Paramyxovirus group). It spreads through droplets released when an infected person coughs or sneezes. Symptoms include fever, cough, rash, and watery eyes. Other diseases listed — scurvy, asthma, and diabetes — are non-communicable as they do not spread from person to person.

The purity of milk is tested using a Lactometer. It works on the principle of Archimedes’ principle and is used to measure the specific gravity (density) of milk. Pure milk has a specific gravity ranging between 1.026 and 1.032. If water or other substances are added, the density decreases, indicating adulteration. The lactometer thus helps determine whether the milk is pure or diluted.

Other options (for context)

• Electrometer: Measures electric potential or charge, not related to milk testing.
• Radiometer: Measures radiant energy or light intensity.
• Refractometer: Used to measure refractive index, commonly for sugar solutions, not milk purity.

Explanation: Uranium-235 (U-235) is the isotope used for the production of atomic energy because it is fissionable. When bombarded with neutrons, U-235 undergoes nuclear fission, releasing a large amount of energy along with additional neutrons, which sustain the chain reaction in nuclear reactors and atomic bombs.

Explanation: The Sun produces its enormous energy through the process of nuclear fusion. In this process, hydrogen nuclei (protons) combine under extremely high temperature and pressure to form helium, releasing a tremendous amount of energy in the form of light and heat. The main fusion reaction in the Sun is the proton–proton chain. Unlike fission (splitting of nuclei), fusion combines light nuclei, and this is what powers all stars, including the Sun.