To solve the question "Which of the following is nitrogen fixing algae?", we need to analyze the options provided and identify which organisms are capable of fixing nitrogen.
### Step-by-Step Solution:
1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain organisms convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds, making nitrogen accessible to living organisms.
2. **Identifying Algae**:
- Algae that can fix nitrogen are primarily cyanobacteria, often referred to as blue-green algae. Examples include Nostoc and Anabaena.
3. **Analyzing the Options**:
- **Option A**: Nostoc, Anabaena, and Oscillatoria.
- Nostoc and Anabaena are known nitrogen-fixing cyanobacteria.
- Oscillatoria is also a cyanobacterium and can fix nitrogen.
- Therefore, this option contains all nitrogen-fixing algae.
- **Option B**: Azolla, Anabaena, and Azotobacter.
- Azolla is a water fern that forms a symbiotic relationship with Anabaena for nitrogen fixation, but it is not an algae itself.
- Azotobacter is a nitrogen-fixing bacterium, not an algae.
- Hence, this option is incorrect.
- **Option C**: Ocellatoria, Anabaena, and Azolla.
- Ocellatoria is a cyanobacterium and can fix nitrogen.
- Anabaena is also a nitrogen-fixing cyanobacterium.
- However, Azolla is not an algae, making this option incorrect.
- **Option D**: Azolla, Nostoc, and Oscillatoria.
- Azolla is not an algae, even though Nostoc and Oscillatoria are nitrogen-fixing cyanobacteria.
- Thus, this option is also incorrect.
4. **Conclusion**:
- The only option that includes organisms that are all nitrogen-fixing algae is **Option A**: Nostoc, Anabaena, and Oscillatoria.
### Final Answer:
**Option A**: Nostoc, Anabaena, and Oscillatoria.
