The correct answer is A. Degradation.
Dissolved oxygen (DO) is the amount of oxygen that is dissolved in water. It is essential for aquatic life, as it is used in respiration. DO levels can be affected by a number of factors, including temperature, salinity, and pollution.
When DO levels fall below a certain point, it can indicate that the water is polluted. This is because pollution can cause the growth of algae, which use up oxygen as they decompose. As a result, the DO levels in the water can drop to zero, which can kill aquatic life.
The zone of degradation is the area in a natural drainage where the DO levels are lowest. This is because this is the area where the most pollution is concentrated. The zone of degradation is often characterized by the presence of dead fish and other aquatic life.
The other options are incorrect because they do not accurately describe the zone of degradation. Option B, active decomposition, is incorrect because decomposition is a process that occurs in all areas of a natural drainage, not just the zone of degradation. Option C, recovery, is incorrect because the zone of degradation is an area where the environment is deteriorating, not improving. Option D, cleaner water, is incorrect because the zone of degradation is an area where the water is polluted, not clean.