The process of "eutrophication" is due to:

Options

A. Increase in concentration of insecticide in water
B. Increase in concentration of fluoride ion in water
C. The reduction in concentration of the dissolved oxygen in water due to phosphate pollution in water
D. Attack of younger leaves of a plant by peroxyacetyl nitrate

Explanation

### Step-by-Step Solution:

1. **Understanding Eutrophication**:
- Eutrophication is a process that occurs in water bodies where there is an excessive accumulation of nutrients, particularly nitrogen and phosphorus. This leads to an overgrowth of algae and aquatic plants.

2. **Causes of Eutrophication**:
- The primary cause of eutrophication is the runoff of fertilizers and other nutrient-rich substances into water bodies. These nutrients promote the rapid growth of algae.

3. **Effects of Eutrophication**:
- As algae grow excessively, they can cover the surface of the water, blocking sunlight from reaching deeper layers. This prevents photosynthesis in aquatic plants below the surface.

4. **Oxygen Depletion**:
- When the algae die, they are decomposed by bacteria, a process that consumes a significant amount of dissolved oxygen in the water. This leads to hypoxic conditions (low oxygen levels), which can be detrimental to fish and other aquatic life.

5. **Water Quality Degradation**:
- The overall quality of the water deteriorates due to the high levels of nutrients and the subsequent depletion of oxygen. This can lead to the death of fish and other aquatic organisms, creating a cycle of environmental degradation.

6. **Conclusion**:
- Therefore, the process of eutrophication is primarily due to the excessive input of nutrients, particularly phosphates, into water bodies, leading to algal blooms and subsequent oxygen depletion.