_______is a free-living `N_(2)`-fixing aerobic bacterium.

Options

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

Answer

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks for a free-living nitrogen-fixing aerobic bacterium. This means we need to identify a bacterium that can fix nitrogen from the atmosphere, does not require a symbiotic relationship with other organisms, and thrives in the presence of oxygen.

2. **Define Key Terms**:
- **Free-living**: Refers to organisms that do not depend on other organisms for their survival and can live independently.
- **Nitrogen-fixing**: This refers to the ability of certain bacteria to convert atmospheric nitrogen (N₂) into a form that plants can use, such as ammonia.
- **Aerobic**: This term indicates that the organism requires oxygen for its metabolic processes.

3. **Evaluate the Options**:
- **Rhodospirillum**: This bacterium is free-living but is anaerobic, meaning it does not require oxygen for survival. Thus, it does not fit the criteria.
- **Azotobacter**: This bacterium is known to be free-living, nitrogen-fixing, and aerobic. It fits all the criteria provided in the question.
- **Clostridium**: Similar to Rhodospirillum, Clostridium is free-living but is anaerobic, so it does not meet the requirements.
- **Rhizobium**: This bacterium is symbiotic, meaning it lives in association with leguminous plants and forms root nodules. Therefore, it does not qualify as free-living.

4. **Conclusion**: Based on the evaluation of the options, the correct answer is **Azotobacter**, as it is the only bacterium that is free-living, nitrogen-fixing, and aerobic.

5. **Final Answer**: Azotobacter is a free-living nitrogen-fixing aerobic bacterium.

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