Explanation: According to the Bureau of Indian Standards (BIS) – IS 10500:2012, the acceptable limit of total hardness in drinking water, expressed as CaCO₃, is 200 mg/l. The permissible limit in the absence of an alternate source is up to 600 mg/l. Excess hardness can cause scaling and affect the taste of water.

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.

Green leafy vegetables are a rich source of Folic Acid (Vitamin B₉). Folic acid is essential for the formation of red blood cells, DNA synthesis, and proper cell growth. It is particularly important for pregnant women, as it helps prevent neural tube defects in the developing fetus. Vegetables like spinach, kale, lettuce, and fenugreek leaves are excellent sources of folic acid.

Other options (for context)

• Citric acid: Found mainly in citrus fruits like oranges and lemons.
• Malic acid: Commonly present in apples and other fruits, not in leafy greens.
• None of these: Incorrect — folic acid is the correct nutrient associated with green leafy vegetables.

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: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

Explanation: The elements Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) — collectively known as CHON — were primarily responsible for the origin of life on Earth. These elements are the basic building blocks of all biological molecules such as proteins, nucleic acids, carbohydrates, and lipids. Early Earth's atmosphere contained these elements in gaseous forms (like CH₄, NH₃, H₂, and H₂O), which combined under lightning and UV radiation to form simple organic compounds — a process explained by the Miller–Urey experiment.

The aorta is the largest artery in the human body. It originates from the left ventricle of the heart and distributes oxygenated blood to all parts of the body through its various branches. The aorta has thick, elastic walls that can withstand the high pressure of blood pumped directly from the heart. It is divided into parts such as the ascending aorta, aortic arch, and descending aorta.

Other options (for context)

• Capillary: The smallest blood vessels where exchange of gases and nutrients occurs.
• Vena cava: The largest vein, not an artery; it carries deoxygenated blood back to the heart.
• Pulmonary vein: Carries oxygenated blood from the lungs to the heart but is not the largest vessel.