Nitrogen fixation is performed by

Options

A. Green algae and fungi
B. Ferns and cycads
C. Legumes and cereals
D. Blue-green algae and bacteria.

Answer

### Step-by-Step Text Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds, making it available for plants to absorb and utilize.

2. **Role of Microorganisms**:
- This process is primarily carried out by certain microorganisms, including specific bacteria and cyanobacteria (also known as blue-green algae). These organisms can either live freely in the soil or in symbiotic relationships with plants.

3. **Identifying the Organisms**:
- The key organisms involved in nitrogen fixation include:
- **Bacteria**: Such as Rhizobia (which form symbiotic relationships with legumes) and Azotobacter (which can live freely).
- **Cyanobacteria**: Such as Nostoc and Anabaena, which can also fix nitrogen either freely or in association with plants.

4. **Evaluating the Options**:
- **Option A**: Green algae and fungi - Incorrect, as these are eukaryotic organisms and do not perform nitrogen fixation.
- **Option B**: Ferns and cycads - Incorrect, as these are plants that require nitrogen fixation from microorganisms.
- **Option C**: Legumes and cereals - Incorrect, as these are also plants that depend on nitrogen-fixing microorganisms.
- **Option D**: Blue-green algae and bacteria - Correct, as both cyanobacteria and bacteria are known to fix nitrogen.

5. **Conclusion**:
- The correct answer is **Option D: Blue-green algae and bacteria**, as they are the organisms that perform nitrogen fixation.