Oxygen demanding wastes

The correct option is Decrease the oxygen level in water.

Explanation:

Oxygen-demanding wastes refer to organic substances (such as sewage, agricultural runoff, and industrial waste) that can be decomposed by aerobic microorganisms (bacteria) present in water bodies.
When these organic wastes enter an aquatic ecosystem, the population of aerobic bacteria increases rapidly as they consume the waste as a food source.

To break down this organic matter, these bacteria require dissolved oxygen (DO) from the water for their cellular respiration. As the decomposition process intensifies, the bacteria consume dissolved oxygen at a rate much faster than it can be replenished from the atmosphere or through photosynthesis by aquatic plants.
Consequently, the concentration of dissolved oxygen in the water body decreases significantly, a process often quantified as an increase in Biochemical Oxygen Demand (BOD). This depletion of oxygen can lead to hypoxic conditions, severely harming or killing fish and other aquatic organisms that depend on dissolved oxygen to survive.