A free living as well as symbiotic nitrogen-fixing prokaryote is:

Options

A. Spirogyra
B. Anabaena
C. Oedogonium
D. Cladophora

Explanation

To determine a free-living as well as symbiotic nitrogen-fixing prokaryote, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process where atmospheric nitrogen (N2) is converted into ammonia (NH3) or related compounds. This is crucial because plants cannot directly utilize atmospheric nitrogen.

2. **Identifying Prokaryotes**:
- Prokaryotes are single-celled organisms that lack a nucleus. They include bacteria and archaea. We are looking for a specific type of prokaryote that can fix nitrogen both in free-living conditions and in a symbiotic relationship.

3. **Types of Nitrogen-Fixing Prokaryotes**:
- There are two categories:
- **Free-living nitrogen-fixers**: These organisms live independently in the soil and can fix nitrogen. An example is Azotobacter.
- **Symbiotic nitrogen-fixers**: These organisms form a mutualistic relationship with plants. An example is Rhizobium, which associates with leguminous plants.

4. **Identifying the Correct Organism**:
- We need to find a prokaryote that fits both categories.
- Anabena, a genus of cyanobacteria, is known to fix nitrogen and can exist both freely in aquatic environments and in symbiotic relationships with certain plants and ferns.

5. **Conclusion**:
- Based on the above analysis, the correct answer is **Anabena**, as it is a free-living as well as a symbiotic nitrogen-fixing prokaryote.

### Final Answer:
The free-living as well as symbiotic nitrogen-fixing prokaryote is **Anabena**.

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