Zone of Recovery is the third zone of pollution in the river. This zone is characterized by the gradual improvement in water quality and ecosystem health after a period of pollution or degradation. It is the stage where the river begins to heal and recover from the impacts of pollution.

Here is an explanation of why Zone of Recovery is the correct answer:

1. Zone of Recovery:

- This zone occurs after the initial pollution event or degradation in the river. It is characterized by a decrease in pollutant concentrations and an improvement in water quality.
- During this phase, natural processes such as dilution, sedimentation, and biological degradation help to reduce the pollutant levels in the water.
- The river ecosystem starts to recover as the pollution load decreases, and the natural flora and fauna gradually return to the area.
- The water quality parameters, such as dissolved oxygen levels, pH, and nutrient concentrations, begin to stabilize and approach their normal ranges.
- The recovery of the river ecosystem may take a considerable amount of time, depending on the severity and duration of the pollution event.

2. Zone of Degradation:

- The zone of degradation is the second stage of pollution in the river.
- It is characterized by high pollutant concentrations, poor water quality, and a significant decline in the health of the ecosystem.
- In this zone, the impacts of pollution are clearly visible, with dead or dying aquatic life, foul odor, and discolored water.
- The water may contain high levels of toxic substances, organic matter, and nutrients, leading to eutrophication and oxygen depletion.
- The degradation may result from various sources, such as industrial discharges, agricultural runoff, or improper waste disposal.

3. Zone of Clean Water:

- The zone of clean water is the ideal state of a river where there is no pollution or degradation.
- It is characterized by pristine water quality, abundant and diverse aquatic life, and a healthy ecosystem.
- In this zone, the water meets all the required water quality standards and supports various activities such as drinking water supply, recreation, and aquatic habitat.

4. Zone of Active Decomposition:

- The zone of active decomposition is a stage where organic matter and pollutants break down through microbial activity.
- It is a natural process that occurs in rivers and other aquatic environments, where bacteria and other microorganisms decompose organic materials, releasing nutrients and energy.
- This decomposition process can lead to changes in water quality, such as fluctuations in dissolved oxygen levels and nutrient concentrations.

In conclusion, the third zone of pollution in the river is the Zone of Recovery. This stage represents the gradual improvement in water quality and ecosystem health after a period of pollution or degradation. It is an important phase where natural processes help to restore the river to a healthier state.