Biofertiliser is

Options

A. (a) farm yard manure comprising a mixture of cattle dung and crop residue
B. (b) green manure in which quick growing crop is cultivated and ploughed under the soil
C. (c) a cyanobacteria like Anabaena species living in the cavities of Azolla species
D. (d) symbiotic bacteria like Azotobacter which fix atmospheric nitrogen

Answer

### Step-by-Step Solution

1. **Understanding Biofertilizers**:
- Biofertilizers are substances that contain living microorganisms which promote plant growth by increasing the supply or availability of primary nutrients to the host plant. They are derived from natural sources.

2. **Components of Biofertilizers**:
- The term "biofertilizer" is made up of two parts: "bio" (meaning life) and "fertilizer" (substances that provide nutrients to plants). Therefore, biofertilizers are fertilizers that consist of living organisms.

3. **Function of Biofertilizers**:
- Biofertilizers enhance the nutrient content in the soil, particularly nitrogen and phosphorus, which are essential for healthy plant growth. They help in making these nutrients more accessible to plants.

4. **Types of Microorganisms in Biofertilizers**:
- Biofertilizers mainly consist of microorganisms such as bacteria, fungi, and algae. These microorganisms can exist in two forms:
- **Symbiotic**: They have a mutualistic relationship with plants, where both organisms benefit. For example, certain bacteria live in the roots of plants and help in nitrogen fixation.
- **Free-living**: These microorganisms do not depend on plants for survival but still provide nutrients to them.

5. **Evaluating Options**:
- **Option A**: Farmyard manure (cattle dung and crop residue) is organic but does not contain living microorganisms specifically for nutrient enhancement, so it is not classified as a biofertilizer.
- **Option B**: Green manure involves ploughing under quick-growing crops, but it primarily consists of organic matter and does not specifically refer to microorganisms, making it incorrect as a biofertilizer.
- **Option C**: Cyanobacteria like Anabena species, which live in a symbiotic relationship with Azolla species, are indeed biofertilizers because they fix atmospheric nitrogen and provide it to the plant. This is the correct answer.
- **Option D**: Azotobacter is a free-living nitrogen-fixing bacterium, but it does not fit the definition of a biofertilizer in the context of this question.

6. **Conclusion**:
- The correct answer is **Option C**: Cyanobacteria like Anabena species living in the cavities of Azolla species.