A completely free living organism which takes part in nitrogen fixation is

Options

A. Anabaena
B. Bacillus
C. Azotobacter
D. Rhizobium.

Explanation

To determine the completely free-living organism that participates in nitrogen fixation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain organisms convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds, making nitrogen available for plants.

2. **Identify Types of Nitrogen-Fixing Organisms**:
- There are two main types of nitrogen-fixing organisms:
- **Free-living organisms**: These organisms do not require a host plant to fix nitrogen.
- **Symbiotic organisms**: These organisms live in a mutualistic relationship with plants, often in root nodules.

3. **Examine the Options**:
- **Anabena**: This organism can be both free-living and symbiotic. It is not exclusively free-living.
- **Bacillus**: Some species of Bacillus can fix nitrogen, but they can also be symbiotic. Thus, this option is not exclusively free-living.
- **Azotobacter**: This is a well-known free-living nitrogen-fixing bacterium that does not require a symbiotic relationship with plants.
- **Rhizobium**: This organism is strictly symbiotic, forming root nodules with legumes and is not free-living.

4. **Select the Correct Answer**:
- Based on the analysis, **Azotobacter** is the only organism that is completely free-living and capable of nitrogen fixation. Therefore, the correct answer is **Azotobacter**.

### Final Answer:
The completely free-living organism that participates in nitrogen fixation is **Azotobacter**.

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