An example of free living nitrogen fixing aerobic bacteria is :

Options

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

Explanation

To solve the question "An example of free-living nitrogen-fixing aerobic bacteria is:", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify a free-living nitrogen-fixing aerobic bacterium. This means we are looking for a bacterium that can fix nitrogen from the atmosphere and does not require a host plant to live.

2. **Define Nitrogen Fixation**: Nitrogen fixation is the process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃) by certain bacteria. This is crucial for making nitrogen available to plants.

3. **Identify Types of Nitrogen-Fixing Bacteria**:
- **Free-living Nitrogen Fixers**: These bacteria do not form symbiotic relationships with plants. They can live independently in the soil.
- **Symbiotic Nitrogen Fixers**: These bacteria form partnerships with plants (like legumes) to fix nitrogen.

4. **Consider Aerobic vs. Anaerobic Conditions**:
- **Aerobic Bacteria**: These require oxygen for their metabolic processes.
- **Anaerobic Bacteria**: These do not require oxygen and may even be harmed by it.

5. **List Possible Options**: The options provided in the question may include:
- Clostridium (anaerobic)
- Rhizobium (symbiotic)
- Azotobacter (aerobic and free-living)
- Bejeringia (aerobic and free-living)
- Francia (symbiotic)

6. **Evaluate Each Option**:
- **Clostridium**: This is an anaerobic bacterium, so it does not fit our criteria.
- **Rhizobium**: This is a symbiotic bacterium, so it does not fit our criteria.
- **Azotobacter**: This is a free-living and aerobic nitrogen-fixing bacterium, which fits our criteria.
- **Bejeringia**: This is also a free-living and aerobic nitrogen-fixing bacterium, which fits our criteria.
- **Francia**: This is a symbiotic bacterium, so it does not fit our criteria.

7. **Conclusion**: Since the question asks for an example, both Azotobacter and Bejeringia are correct answers. However, if we are to choose one, Azotobacter is often the most commonly referenced example.

### Final Answer:
An example of free-living nitrogen-fixing aerobic bacteria is **Azotobacter**.