A good green manure in rice fields is

Options

A. Salvinia
B. Azolla
C. Aspergillus
D. Mucor

Answer

### Step-by-Step Solution:

1. **Understanding Green Manure**: Green manure refers to plants that are grown specifically to be plowed back into the soil to improve its fertility. These plants can enhance soil structure, increase organic matter, and provide nutrients.

2. **Identifying Suitable Plants**: The question asks for a good green manure specifically for rice fields. We need to evaluate the options provided: Salvinia, Azolla, Aspergillus, and Muca.

3. **Evaluating Azolla**: Azolla is a type of pteridophyte (a group of seedless vascular plants). It is known for its ability to grow in water and is often found in rice paddies.

4. **Nitrogen Fixation**: Azolla has a symbiotic relationship with a cyanobacterium called Anabaena. This relationship allows Azolla to fix atmospheric nitrogen, converting it into ammonia, which is a form of nitrogen that plants can utilize.

5. **Benefits for Rice Plants**: The ammonia produced from nitrogen fixation is beneficial for rice plants as it enhances their growth and yield. This makes Azolla an excellent choice for green manure in rice fields.

6. **Geographical Relevance**: Azolla is commonly found in regions such as southern China and northern Vietnam, where rice cultivation is prevalent, further supporting its use as green manure in these areas.

7. **Conclusion**: Based on the above evaluation, the best option for a good green manure in rice fields is **Azolla**.

### Final Answer:
**Azolla**

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