Nitrogen fixing cyanobacteria are:-

Options

A. Rhizobium, Nostoc
B. Rhizobium, Anabaena
C. Nostoc , Anabaena
D. Anabaena, Azolla

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into a form that plants can use, such as ammonia (NH₃), nitrates (NO₃⁻), and nitrites (NO₂⁻). This is crucial for soil fertility.

2. **Identifying Nitrogen-Fixing Organisms**:
- Nitrogen-fixing organisms can be categorized into two groups: free-living organisms and symbiotic organisms.
- Free-living nitrogen-fixing organisms can fix nitrogen independently, while symbiotic organisms form partnerships with plants (e.g., Rhizobium with legumes).

3. **Focusing on Cyanobacteria**:
- Cyanobacteria are a group of photosynthetic bacteria known for their ability to fix nitrogen.
- Examples of nitrogen-fixing cyanobacteria include Nostoc and Anabaena.

4. **Analyzing the Options**:
- If the question provides options, we need to identify which of them are cyanobacteria that can fix nitrogen.
- From the information provided, both Nostoc and Anabaena are identified as nitrogen-fixing cyanobacteria.

5. **Conclusion**:
- Based on the analysis, the correct answer to the question "Nitrogen fixing cyanobacteria are:" is the option that includes both Nostoc and Anabaena.

### Final Answer:
The nitrogen-fixing cyanobacteria are **Nostoc and Anabaena**.

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