A green manure is

The correct option is Sesbania.

Step-by-Step Explanation:

1. Understanding Green Manure:
Green manure refers to specific crops that are grown primarily to be plowed back into the soil while still green. The main purpose of this agricultural practice is to improve the soil structure, increase organic matter (humus), and enrich the soil with nutrients, particularly nitrogen.

2. The Role of Leguminous Crops:
Plants chosen for green manure are typically legumes. Leguminous plants have a symbiotic relationship with nitrogen-fixing bacteria (such as Rhizobium) in their root nodules. These bacteria capture atmospheric nitrogen and convert it into a form that plants can easily absorb. When these crops are incorporated back into the soil, they decompose and release a large amount of nitrogen, making it available for subsequent crops.

3. Analyzing the Options:
- Sesbania (commonly known as Dhaincha): It is a fast-growing, leguminous plant widely recognized and used globally as a highly effective green manure crop because of its high nitrogen-fixing capacity and rapid biomass production.
- Maize, Rice, and Sorghum: These are cereal crops belonging to the Poaceae family. They do not fix atmospheric nitrogen and are primary consumer crops that deplete soil nutrients rather than enriching them when grown.

Therefore, among the given choices, Sesbania is the correct example of a green manure crop.