[A]: The nodulated bacteria fix nitrogen in the presence of leg Hb.
[R]: Leg Hb acts as oxygen scavanger

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "the nodulated bacteria fix nitrogen in the presence of leg hemoglobin." This means that the bacteria, specifically rhizobium, which are found in the root nodules of leguminous plants, are capable of fixing nitrogen when leg hemoglobin is present.

2. **Understanding the Reason**: The reason states that "leg hemoglobin acts as an oxygen scavenger." This indicates that leg hemoglobin plays a role in reducing the concentration of oxygen around the nitrogen-fixing bacteria.

3. **Role of Nodulated Bacteria**: Nodulated bacteria, such as rhizobium, are crucial for biological nitrogen fixation. They convert atmospheric nitrogen (N₂) into a form that plants can use (ammonia, NH₃) using an enzyme called nitrogenase.

4. **Sensitivity of Nitrogenase**: The nitrogenase enzyme is sensitive to oxygen. High levels of oxygen inhibit its activity, which means that nitrogen fixation can only occur efficiently in low or no oxygen conditions.

5. **Function of Leg Hemoglobin**: Leg hemoglobin is a pigment found in the root nodules of leguminous plants. It has a high affinity for oxygen and functions as an oxygen scavenger. By binding to oxygen, leg hemoglobin reduces the concentration of free oxygen around the nitrogenase enzyme, allowing nitrogen fixation to occur.

6. **Evaluating the Assertion and Reason**:
- The assertion is true because nodulated bacteria do fix nitrogen in the presence of leg hemoglobin.
- The reason is also true because leg hemoglobin does act as an oxygen scavenger, which is essential for the functioning of the nitrogenase enzyme.

7. **Conclusion**: Both the assertion and reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both statements are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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