A nitrogen - fixing microbe associated with Azolla is :

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a nitrogen-fixing microbe that is associated with Azolla. Azolla is a free-floating aquatic fern known for its symbiotic relationship with certain nitrogen-fixing microorganisms.

2. **Analyzing the Options**:
- **Option A: Anabena**: This is a cyanobacterium that is known to form a symbiotic relationship with Azolla. Anabena is capable of fixing atmospheric nitrogen, making it a suitable candidate.
- **Option B: Spirulina**: While Spirulina is a type of cyanobacterium, it is not specifically associated with Azolla for nitrogen fixation. Therefore, this option is incorrect.
- **Option C: Tolypothrix**: This is another type of blue-green algae that can fix nitrogen, but it does not have a known symbiotic relationship with Azolla. Thus, this option is also incorrect.
- **Option D: Nostoc**: Although Nostoc is a nitrogen-fixing cyanobacterium, it does not specifically associate with Azolla in the same way Anabena does. Therefore, this option is incorrect as well.

3. **Conclusion**: Based on the analysis of the options, the correct answer is **Anabena**, as it is the nitrogen-fixing microbe that is specifically associated with Azolla.

**Final Answer**: Anabena

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