The correct answer is option 'B': Statement ii is correct but i is wrong.

Deoxygenation of water refers to the process of reducing or removing oxygen from water. This can occur due to various factors such as excessive organic matter, chemical pollution, or algal blooms. Deoxygenation can have several impacts on the aquatic environment, and the given statements describe two of these impacts.

i. Conversion of eutrophic water body back into oligotrophic:

- Eutrophic water bodies are characterized by high levels of nutrients, particularly nitrogen and phosphorus, which promote the excessive growth of algae and other aquatic plants.
- This excessive growth leads to the formation of algal blooms, which can cover the water surface and block sunlight from reaching the lower layers of the water body.
- As a result, the submerged plants and algae in the lower layers receive less sunlight and consequently produce less oxygen through photosynthesis.
- When the excessive organic matter produced by the algal blooms decomposes, it consumes the available oxygen in the water, leading to oxygen depletion.
- This deoxygenation of the water body can result in the conversion of a eutrophic water body, which is characterized by high nutrient levels and excessive plant growth, into an oligotrophic water body, which has low nutrient levels and limited plant growth.
- Therefore, statement i is incorrect.

ii. Growth of anaerobic microbes producing secondary pollutants like H2S:

- Deoxygenation of water creates a suitable environment for the growth of anaerobic microbes, which are organisms that can survive and thrive in the absence of oxygen.
- These anaerobic microbes produce secondary pollutants such as hydrogen sulfide (H2S) through their metabolic processes.
- Hydrogen sulfide is a toxic and foul-smelling gas that can have detrimental effects on the aquatic ecosystem and human health.
- It can cause harm to aquatic organisms, including fish and other aquatic animals, by interfering with their respiration and metabolism.
- Additionally, the release of hydrogen sulfide into the atmosphere can contribute to air pollution and create unpleasant odors.
- Therefore, statement ii is correct.

In conclusion, deoxygenation of water can lead to the growth of anaerobic microbes producing secondary pollutants like hydrogen sulfide. However, it does not cause the conversion of a eutrophic water body back into an oligotrophic water body.