Nitrogen fixation is

Options

A. performed only by plants.
B. the oxidation of nitrogen gas.
C. catalyzed by the enzyme nitrogenase.
D. a single-step chemical reaction.

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen gas (N2) into ammonia (NH3). This is essential for plants as they cannot utilize nitrogen gas directly.

2. **Types of Nitrogen Fixation**:
- There are various methods of nitrogen fixation, including biological, industrial, and electrical nitrogen fixation. It is important to note that nitrogen fixation is not limited to plants alone.

3. **Role of Enzymes**:
- The biological process of nitrogen fixation is primarily catalyzed by the enzyme nitrogenase. This enzyme facilitates the conversion of nitrogen gas into ammonia.

4. **Reduction Process**:
- During nitrogen fixation, nitrogen gas (N2) is reduced to ammonia (NH3). This means that nitrogen is gaining electrons, which is a reduction process, rather than being oxidized.

5. **Chemical Reaction Nature**:
- Nitrogen fixation is not a single-step chemical reaction. It involves multiple steps and complex biochemical pathways to convert N2 into NH3.

6. **Conclusion**:
- Based on the above points, the correct statement regarding nitrogen fixation is that it is catalyzed by the enzyme nitrogenase.

### Final Answer:
Nitrogen fixation is catalyzed by the enzyme nitrogenase.

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