Which of the following is a correct statement about nitrogen fixation?

Options

A. Plants convert atmospheric nitrogen to ammonia
B. Ammonia is converted to nitrates, which is the part of nitrogen most easily absorbed by plants
C. Mutant strains of Rhizobium are able to secrete excess nitrogen into the soil
D. Enzyme nitrogenase reduces Na to form ammonia

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds, making nitrogen accessible to plants.

2. **Evaluating the Statements**:
- The first statement claims that plants convert atmospheric nitrogen to ammonia. This is incorrect because it is the bacteria, not the plants, that perform this conversion.
- The second statement suggests that ammonia is converted to nitrates. This is a true statement, as ammonia can be further processed by nitrifying bacteria into nitrates (NO₃⁻), which plants can absorb.
- The third statement mentions mutants of strain Rhizobium secreting excess nitrogen into the soil. This is false; Rhizobium bacteria fix nitrogen but do not secrete it in excess.
- The last statement discusses the enzyme nitrogenase, which is responsible for reducing atmospheric nitrogen (N₂) to ammonia (NH₃). This statement is correct.

3. **Identifying the Correct Statement**:
- Based on the evaluation, the correct statement is that nitrogenase enzymes produced by certain bacteria reduce atmospheric nitrogen (N₂) to form ammonia (NH₃).

4. **Conclusion**:
- The correct answer to the question about nitrogen fixation is that nitrogenase enzymes, produced by specific bacteria, are responsible for the reduction of atmospheric nitrogen to ammonia.