The correct answer is option 'A) Zone of degradation'.

Explanation:
When wastewater is discharged into a river, it can have detrimental effects on the aquatic ecosystem. The zone where fish life might tend to progressively dwindle due to the discharge of wastewater is known as the zone of degradation. Let's understand the concept in detail:

1. Zone of degradation:
- When wastewater is discharged into a river, it introduces various pollutants such as organic matter, nutrients, chemicals, and pathogens into the water.
- The excessive organic matter and nutrients lead to an increase in the biological oxygen demand (BOD) and chemical oxygen demand (COD) of the water.
- As a result, the oxygen levels in the water start to decrease because bacteria and other microorganisms consume oxygen while decomposing the organic matter.
- This decrease in oxygen levels can be harmful to fish and other aquatic organisms, as they require oxygen to survive.
- In the zone of degradation, the fish population starts to dwindle gradually as they struggle to survive in the oxygen-depleted water.
- The decrease in fish population can disrupt the balance of the aquatic ecosystem and affect other organisms dependent on fish as a food source.

2. Zone of active decomposition:
- This zone refers to the area where the wastewater and its pollutants are actively decomposing.
- As the organic matter decomposes, it releases nutrients like nitrogen and phosphorus, which can lead to eutrophication.
- Eutrophication is a process where excessive nutrients cause an overgrowth of algae and aquatic plants, leading to oxygen depletion.

3. Zone of mixing:
- This zone is where the wastewater mixes with the river water.
- The mixing of wastewater dilutes its concentration of pollutants, but it also spreads the pollutants over a larger area.

4. Zone of recovery:
- This zone refers to the area downstream where the river starts to recover from the impacts of the wastewater discharge.
- The recovery process includes natural purification mechanisms such as dilution, sedimentation, filtration, and biological degradation.
- In the zone of recovery, the oxygen levels gradually increase, allowing the fish and other organisms to thrive again.

Conclusion:
The zone of degradation is the area where fish life might progressively dwindle when wastewater is discharged into a river. The introduction of pollutants, especially excessive organic matter and nutrients, leads to oxygen depletion, making it difficult for fish to survive. This highlights the importance of proper wastewater treatment and management to protect the health of aquatic ecosystems.