A water fern capable of fixing atmospheric nitrogen and used as biofertilizer.

Options

A. Azolla
B. Nostoc
C. Adiantum
D. Spirulina

Answer

To solve the question regarding a water fern capable of fixing atmospheric nitrogen and used as a biofertilizer, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Options**: The question provides four options: Azolla, Nostoc, Adiantum, and Spirulina.

2. **Analyze Each Option**:
- **Azolla**: This is a genus of water ferns that has a symbiotic relationship with nitrogen-fixing cyanobacteria (specifically, Anabaena). This allows Azolla to fix atmospheric nitrogen, making it a valuable biofertilizer.
- **Nostoc**: This is a genus of cyanobacteria, not a fern. While it can fix nitrogen, it does not fit the description of a water fern.
- **Adiantum**: This is a genus of ferns but does not have the capability to fix atmospheric nitrogen.
- **Spirulina**: This is a type of blue-green algae (cyanobacteria) and is not a fern. It is also known for its nutritional benefits but does not fit the description.

3. **Conclusion**: Based on the analysis, Azolla is the only option that is a water fern capable of fixing atmospheric nitrogen and is used as a biofertilizer.

4. **Final Answer**: The correct answer is **Azolla**.