Step 1: Understanding the Concept:
Green manure is any plant matter grown on purpose and then mixed back into the soil while still green, so it rots down and adds organic matter plus nitrogen to the field. Farmers pick fast-growing plants for this job so the soil gets enriched quickly between crop cycles.
Step 2: Key Approach:
The best green manure plants either grow very fast or carry nitrogen-fixing partners that pull nitrogen gas from the air and turn it into a form plants can use. Checking each option against this idea points to the right choice.
Step 3: Detailed Explanation:
Azolla is a small floating water fern that houses the nitrogen-fixing cyanobacterium Anabaena azollae in its leaf cavities. Rice farmers grow Azolla on the flooded paddy field itself, then plough it into the mud, where it decomposes fast and releases fixed nitrogen right where the rice needs it. This makes it one of the most common green manure sources used specifically in wetland rice farming. Now the other options: Azadirachta indica (neem) is valued for its pest-repelling compounds, not as a soil-enriching crop. Hevea brasiliensis is the rubber tree, grown for latex, with no role in manuring. Crotalaria juncea (sunn hemp) is also a genuine green manure legume, but the question is pointing at the aquatic, paddy-specific option, which is Azolla.
Step 4: Final Answer:
Azolla is grown and ploughed into paddy soil as green manure because of its Anabaena partnership, so option 2 is correct.