During biological nitrogen fixation, inactivation of nitrogenase by oxygen poisoning is prevented by

Options

A. Xanthophyll
B. Carotene
C. Cytochrome
D. Leghaemoglobin

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃), which can be utilized by plants. This process is crucial because most organisms cannot directly use atmospheric nitrogen.

2. **Role of Nitrogenase**:
- The enzyme responsible for nitrogen fixation is called nitrogenase. It catalyzes the conversion of nitrogen gas into ammonia. However, nitrogenase is highly sensitive to oxygen.

3. **Oxygen Poisoning**:
- In the presence of molecular oxygen (O₂), nitrogenase becomes inactivated, which halts the nitrogen fixation process. Therefore, it is essential to prevent oxygen from interfering with nitrogenase activity.

4. **Adaptations to Prevent Oxygen Poisoning**:
- Certain organisms, particularly those involved in biological nitrogen fixation, have adaptations to mitigate the effects of oxygen. One such adaptation is found in root nodules of leguminous plants.

5. **Role of Leghemoglobin**:
- In the root nodules, a protein called leghemoglobin is present. Leghemoglobin acts as an oxygen scavenger. It binds to oxygen, thereby reducing its availability to nitrogenase and preventing the inactivation of this enzyme.

6. **Conclusion**:
- Therefore, during biological nitrogen fixation, the inactivation of nitrogenase by oxygen poisoning is prevented by leghemoglobin.

### Final Answer:
The correct answer is **leghemoglobin**.

---