A free living anaerobic bacterium capable of fixing nitrogen is

Options

A. Azotobacter
B. Rhizobium
C. Clostridium
D. Streptococcus.

Answer

To solve the question "A free living anaerobic bacterium capable of fixing nitrogen is," we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a bacterium that is both free-living and anaerobic, and has the capability to fix nitrogen.

2. **Identify Nitrogen-Fixing Bacteria**: Nitrogen-fixing bacteria can be categorized into two types: free-living and symbiotic (non-free-living). Free-living bacteria can fix nitrogen independently, while symbiotic bacteria, like Rhizobium, depend on a host plant.

3. **List the Options**: The options provided are:
- Option 1: Azotobacter
- Option 2: Rhizobium
- Option 3: Clostridium
- Option 4: Streptococcus

4. **Evaluate Each Option**:
- **Azotobacter**: This is a free-living nitrogen-fixing bacterium, but it is aerobic, not anaerobic.
- **Rhizobium**: This is a symbiotic nitrogen-fixing bacterium associated with legumes, hence not free-living.
- **Clostridium**: This is a free-living anaerobic bacterium that can fix nitrogen.
- **Streptococcus**: This is not known for nitrogen fixation.

5. **Conclusion**: Based on the evaluation, the only bacterium that fits the criteria of being a free-living anaerobic nitrogen fixer is **Clostridium**.

### Final Answer:
The correct answer is **Clostridium** (Option 3).

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