The Zone of Degradation

The river in question is experiencing pollution, as indicated by the fact that the algae are unable to survive. The presence of fishes suggests that the pollution is not severe enough to completely eliminate all aquatic life, but it is still impacting the ecosystem.

Definition of the Zone of Degradation
The zone of degradation refers to an area within a water body where pollution levels have reached a point where the natural ecosystem is adversely affected. In this zone, the water quality is significantly compromised, leading to a decline in the health and abundance of aquatic plants and animals.

Explanation of the Correct Answer
The correct answer to this question is option 'B', the zone of degradation. This is because the river is polluted to an extent that algae, which are typically present in healthy aquatic ecosystems, are unable to survive. The presence of fishes indicates that the pollution has not yet completely eliminated all life in the river, but it has significantly impacted the ecosystem.

Other Options Explained
- Option 'A', the zone of recovery, refers to an area where efforts are being made to restore the health and quality of the water body. In this case, since the pollution is still ongoing and the algae are unable to survive, it is not the zone of recovery.
- Option 'C', the zone of clean water, suggests an area where the water quality is unaffected by pollution and remains in a pristine condition. Since the river in question is polluted, it is not the zone of clean water.
- Option 'D', the zone of active decomposition, typically refers to an area where organic matter in the water undergoes decomposition, leading to a decrease in dissolved oxygen levels. While the presence of fishes suggests that some decomposition may be occurring, the primary issue in this case is pollution rather than decomposition.

In conclusion, the correct answer is option 'B', the zone of degradation, as the river is polluted to a level where algae are unable to survive, indicating a significant negative impact on the ecosystem.