Which of the following is nitrogen fixing

Options

A. Nostoc, Anabaena, Oscillatoria
B. Azolla, Anabaenra, Azotobactes
C. Oscillatoria, Anabaena, Azolla
D. Azolla, Nostoc, Oscillatoria

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds in soil, making nitrogen available to plants.

2. **Identifying Nitrogen-Fixing Organisms**:
- The question asks for organisms that can fix nitrogen. We need to identify which of the given options are known for this ability.

3. **Analyzing the Options**:
- The options provided include various organisms. We need to focus on the ones that are known to be nitrogen-fixing bacteria.

4. **Recognizing Cyanobacteria**:
- Nostoc, Anabaena, and Oscillatoria are all classified as cyanobacteria (also known as blue-green algae). They are capable of photosynthesis and can fix nitrogen.

5. **Confirming Nitrogen Fixation in Cyanobacteria**:
- Some species of Nostoc and Anabaena can fix nitrogen in both free-living and symbiotic forms, while Oscillatoria has some species that can also fix nitrogen.

6. **Eliminating Incorrect Options**:
- Other organisms mentioned (like Azolla) may not fix nitrogen themselves but can associate with nitrogen-fixing bacteria. Therefore, they are not the correct answer.

7. **Conclusion**:
- Based on the analysis, the correct answer to the question is option 1: Nostoc, Anabaena, and Oscillatoria.

### Final Answer:
The nitrogen-fixing organisms are: **Nostoc, Anabaena, and Oscillatoria** (Option 1).

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