A free-living nitrogen-fixing cyanobacterium which can also form symbiotic association with the water fern Azolla is :

Options

A. Chlorella
B. Nostoc
C. Anabaena
D. Tolypothrix

Answer

To answer the question about the free-living nitrogen-fixing cyanobacterium that forms a symbiotic association with the water fern Azolla, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Organism**: The question asks for a cyanobacterium that has a specific relationship with Azolla. We need to recall which cyanobacteria are known for such associations.

2. **Recall Characteristics of Azolla**: Azolla is a free-floating water fern that thrives in freshwater environments. It is known for its ability to form symbiotic relationships with certain cyanobacteria.

3. **Determine the Cyanobacterium**: The cyanobacterium that is commonly associated with Azolla is Anabaena. Anabaena is known for its ability to fix atmospheric nitrogen, making it beneficial for plants.

4. **Explain the Symbiotic Relationship**: In this symbiotic relationship, Anabaena provides fixed nitrogen to Azolla, which is essential for its growth. In return, Azolla offers carbohydrates and other organic compounds to Anabaena, which helps it thrive.

5. **Conclude the Answer**: Based on the information gathered, the answer to the question is Anabaena, as it is the cyanobacterium that forms a symbiotic association with Azolla.

### Final Answer:
The free-living nitrogen-fixing cyanobacterium that forms a symbiotic association with the water fern Azolla is **Anabaena**.

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