Which aquatic fern performs nitrogen fixation :

Options

A. Azolla
B. Nostoc
C. Salvia
D. Salvinia

Answer

To answer the question "Which aquatic fern performs nitrogen fixation?", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- A. Azolla
- B. Nostoc
- C. Salvia
- D. Salvina

2. **Understand Nitrogen Fixation**: Nitrogen fixation is the process by which nitrogen in the atmosphere is converted into ammonia or related compounds in soil and water. This process is crucial for plant growth as it provides essential nutrients.

3. **Review the Options**:
- **Azolla**: This is an aquatic fern that has a symbiotic relationship with blue-green algae (Anabaena), which helps in nitrogen fixation.
- **Nostoc**: This is a genus of blue-green algae that can fix nitrogen but is not classified as an aquatic fern.
- **Salvia**: This is a genus of plants that does not perform nitrogen fixation.
- **Salvina**: This is another type of aquatic plant but does not have a known role in nitrogen fixation.

4. **Focus on Azolla**: Azolla is specifically known for its role in agriculture, particularly in rice paddies, where it helps to enrich the soil with nitrogen through its association with Anabaena.

5. **Conclusion**: Based on the information, the aquatic fern that performs nitrogen fixation is **Azolla**.

### Final Answer:
The correct answer is **A. Azolla**.

---