Nitrogen fixing cyanobacterium present in fern Azolla is

Options

A. Nostoc
B. Trichodesmium
C. Anabaena
D. Oscillatoria

Answer

To answer the question about the nitrogen-fixing cyanobacterium present in the fern Azolla, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Fern**: The question specifies the fern Azolla. Azolla is a small aquatic fern that has a symbiotic relationship with certain cyanobacteria.

2. **Understand the Symbiotic Relationship**: Azolla hosts cyanobacteria in its tissues, which play a crucial role in nitrogen fixation. This means that these cyanobacteria can convert atmospheric nitrogen into a form that the plant can use.

3. **Examine the Cyanobacterium**: The specific cyanobacterium associated with Azolla is known for its filamentous structure. Under a microscope, you can observe these filaments.

4. **Identify the Cyanobacterium**: The cyanobacterium present in Azolla is called **Anabaena**. It is known for its ability to fix nitrogen and is found within specialized cavities in the Azolla plant.

5. **Evaluate Other Options**: The question may provide other options like Nostoc, Trichodesmium, and Oscillatoria. However, these do not have the same symbiotic relationship with Azolla as Anabaena does.

6. **Conclusion**: Therefore, the nitrogen-fixing cyanobacterium present in the fern Azolla is **Anabaena**.

### Final Answer:
The nitrogen-fixing cyanobacterium present in fern Azolla is **Anabaena**.