Which of the following serve as biofertilizer in paddy fields?

Options

A. Bacteria
B. Yeast
C. Cyanobacteria. (blue-green algae)
D. Fungi

Answer

### Step-by-Step Solution:

1. **Understanding Biofertilizers**: Biofertilizers are natural substances that enhance soil fertility and promote plant growth by increasing the availability of nutrients. They typically consist of living microorganisms.

2. **Identifying the Role of Microorganisms**: In the context of paddy fields, certain microorganisms are particularly beneficial. These include bacteria and fungi that can fix nitrogen and improve soil health.

3. **Recognizing Key Microorganisms**: Among the various microorganisms, cyanobacteria (also known as blue-green algae) are significant. They are capable of fixing atmospheric nitrogen, which is crucial for plant growth.

4. **Cyanobacteria in Paddy Fields**: Cyanobacteria thrive in wet environments like paddy fields. They not only enhance soil fertility by fixing nitrogen but also help in maintaining soil pH and improving nutrient availability.

5. **Conclusion**: Based on the role of cyanobacteria in enhancing soil fertility and their specific application in paddy fields, the correct answer to the question is **cyanobacteria**.

### Final Answer:
Cyanobacteria serve as biofertilizers in paddy fields.

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