ANS:B - active decomposition

1. Degradation: When dissolved oxygen concentration falls to zero in a natural drainage, it indicates degradation. Degradation refers to the process by which organic matter in the water undergoes decomposition, often in the absence of oxygen. This anaerobic decomposition leads to the breakdown of organic material, but it also produces byproducts such as hydrogen sulfide, methane, and other foul-smelling compounds. Water in a degraded state may have unpleasant odors, murky appearance, and reduced biodiversity.
2. Active Decomposition: This term refers to the ongoing breakdown of organic matter within the natural drainage system. When dissolved oxygen levels are low or depleted, it indicates an environment where decomposition processes are actively taking place. Anaerobic microorganisms thrive in such conditions, breaking down organic matter through fermentation and other anaerobic processes.
3. Recovery: If dissolved oxygen levels fall to zero, it's unlikely that the water body is in a state of recovery. Recovery typically implies a return to a healthier state after a period of degradation or pollution. In this context, recovery would involve measures to improve dissolved oxygen levels and restore ecological balance within the drainage system, such as reducing pollutant inputs, enhancing oxygenation, and restoring natural habitats.
4. Cleaner Water: Zero dissolved oxygen levels are not indicative of cleaner water. On the contrary, they suggest poor water quality, likely due to pollution, organic matter overload, or other factors leading to oxygen depletion. Clean water typically has adequate dissolved oxygen levels to support aquatic life and maintain ecological balance.
5. None of these: While some of the options may partially apply, the most accurate description of the situation where dissolved oxygen falls to zero in a natural drainage would be degradation, as it reflects the deterioration of water quality and ecological health due to oxygen depletion and anaerobic conditions.