A completely free living organism which takes part in N.-fixation is

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into nitrogenous compounds that plants can use for growth. This is essential because plants cannot utilize nitrogen in its gaseous form.

2. **Identifying Free Living Organisms**:
- A completely free living organism refers to those that can fix nitrogen independently, without forming symbiotic relationships with other organisms.

3. **Types of Nitrogen Fixation**:
- Nitrogen fixation can occur through physical methods (like lightning) or biological methods (through microorganisms). The focus here is on biological nitrogen fixation.

4. **Examples of Nitrogen-Fixing Organisms**:
- There are various microorganisms that can fix nitrogen. Some are free living, while others are symbiotic. In this case, we are looking for a free living organism.

5. **Identifying the Correct Organism**:
- Among the free living nitrogen-fixing organisms, *Azotobacter* is the most common example. It is known for its ability to fix nitrogen from the atmosphere into forms that can be utilized by plants.

6. **Conclusion**:
- Therefore, the completely free living organism that participates in nitrogen fixation is *Azotobacter*.

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