A free living aerobic organism that helps in N, fixation is Azotobacter/Clostridium/Rhizobium.

Explanation

To determine which organism is a free-living aerobic nitrogen fixer among the options provided (Azotobacter, Clostridium, and Rhizobium), we can follow these steps:

### Step-by-Step Solution:

1. **Understand Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into a form that plants can use, such as ammonia or nitrates.

2. **Identify the Organisms**:
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