A biofertilizer is

Options

A. Cyanobacterium Anabaena in cavities of Azolla leaves
B. Symbiotic associtation of Azotobacter for fixing atmospheric nitrogen
C. Farmyard manure
D. Green manure

Answer

### Step-by-Step Solution:

1. **Understanding Biofertilizers**:
- A biofertilizer is a substance that contains living microorganisms, which promote plant growth by increasing the supply or availability of primary nutrients to the host plant.

2. **Identifying Key Components**:
- The question mentions "cyanobacterium anabene" and "Azolla leaves". Here, Anabaena is a type of cyanobacteria known for its nitrogen-fixing ability.

3. **Analyzing the Relationship**:
- Anabaena lives in the cavities of Azolla leaves, forming a symbiotic relationship. This means that Anabaena provides the plant with fixed nitrogen, while the plant provides a habitat and nutrients for the cyanobacterium.

4. **Evaluating the Options**:
- **Option 1**: Cyanobacterium Anabaena in the cavities of Azolla leaves - This is a correct example of a biofertilizer.
- **Option 2**: Symbiotic association of Azotobacter - Azotobacter is free-living and does not form a symbiotic relationship, making this option incorrect.
- **Option 3**: Farmyard manure - While it is beneficial for soil fertility, it is not classified as a biofertilizer because it does not contain living microorganisms that specifically enhance nutrient availability.
- **Option 4**: Green manure - Similar to farmyard manure, it is not a biofertilizer as it does not involve living microorganisms in the same context.

5. **Conclusion**:
- The correct answer is **Option 1**, as it accurately describes a biofertilizer that involves living organisms (Anabaena) that enhance soil fertility through nitrogen fixation.