One of the free nitrogen - fixed in paddy fields is :

Options

A. Beijernickia
B. Rhodospirillum
C. Rhizobium
D. Azotobacter

Answer

To solve the question "One of the free nitrogen-fixed in paddy fields is:", we will analyze the options provided and determine which one correctly identifies a nitrogen-fixing bacteria found in paddy fields.

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be utilized by plants. This process can occur through free-living bacteria or symbiotic bacteria.

2. **Analyzing the Options**:
- **Option 1: Vizernikia**:
- This is a free nitrogen-fixing aerobic microbe, but it is not specifically known for its role in paddy fields. Therefore, this option is incorrect.

- **Option 2: Rhodospirillium**:
- This is a facultative anaerobic bacterium that can fix nitrogen. It is known to be found in paddy fields, making this option a strong candidate for the correct answer.

- **Option 3: Rhizobium**:
- Rhizobium forms symbiotic relationships with legumes and is not a free-living nitrogen fixer in paddy fields. Thus, this option is incorrect.

- **Option 4: Azotobacter**:
- Azotobacter is a free-living aerobic bacterium, but it is not specifically associated with paddy fields. Therefore, this option is also incorrect.

3. **Conclusion**:
- After analyzing all the options, the correct answer is **Option 2: Rhodospirillium**, as it is a free nitrogen-fixing bacterium that can be found in paddy fields.

### Final Answer:
**Rhodospirillium**