In a water body, too much sewage leads to

Correct answer: An increase in the bacterial population which reduces the level of oxygen in the water

Explanation: Sewage is rich in organic matter. Bacteria multiply rapidly to decompose this material, a process that uses up dissolved oxygen (high Biological Oxygen Demand, BOD). As oxygen levels drop, fish and other aquatic animals can suffocate and die.

• A decrease in the temperature… — Incorrect. Sewage typically affects oxygen levels, not by cooling the water.
• An increase in the number of aquatic animals… — Incorrect. Oxygen depletion harms, not boosts, aquatic life.
• An increase in the bacterial population which reduces the level of oxygen… — Correct.
• A decrease in the bacterial population which increases oxygen… — Incorrect. The opposite happens.