An aerobic nitrogen fixing bacterium is

Options

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

Answer

To answer the question "An aerobic nitrogen-fixing bacterium is," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question is asking for an example of an aerobic nitrogen-fixing bacterium. This means we need to identify a bacterium that can fix nitrogen and requires oxygen for its metabolic processes.

2. **Identify Nitrogen-Fixing Bacteria**: Nitrogen-fixing bacteria are capable of converting atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by plants. There are two main types of nitrogen-fixing bacteria: aerobic (requiring oxygen) and anaerobic (not requiring oxygen).

3. **List Known Nitrogen-Fixing Bacteria**: Some well-known nitrogen-fixing bacteria include:
- Azotobacter (aerobic)
- Clostridium (anaerobic)
- Rhodospirillum (anaerobic)
- Chlorobium (anaerobic)

4. **Determine the Aerobic Bacterium**: From the list, Azotobacter is recognized as a free-living nitrogen-fixing bacterium that is aerobic, meaning it thrives in the presence of oxygen.

5. **Eliminate Anaerobic Options**: The other options (Clostridium, Rhodospirillum, and Chlorobium) are all anaerobic bacteria, which means they do not require oxygen and are not suitable answers for the question.

6. **Conclude the Answer**: Based on the above analysis, the correct answer to the question is Azotobacter, as it is the only aerobic nitrogen-fixing bacterium listed.

### Final Answer:
An aerobic nitrogen-fixing bacterium is **Azotobacter**.

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