Green manure plants are

The correct option is Leguminosae.

Green manure refers to crops that are grown specifically to be plowed back and incorporated into the soil while still green. This agricultural practice is designed to improve the soil's physical structure, organic matter content, and nutrient levels.
Plants belonging to the family Leguminosae (commonly known as legumes) are the most preferred choice for green manuring. Examples of these plants include sunn hemp (Crotalaria juncea), sesbania (Dhaincha), clover, and alfalfa.

The key reason why leguminous plants are highly effective as green manure is their symbiotic relationship with nitrogen-fixing bacteria, such as Rhizobium, which live in their root nodules. These bacteria convert atmospheric nitrogen (N₂), which plants cannot directly use, into ammonia and other organic nitrogen compounds that are easily absorbed by plants.

When these plants are plowed back into the field, they decompose rapidly. This decomposition releases the accumulated nitrogen and organic matter directly into the soil, thereby enriching it and reducing the need for chemical fertilizers. In contrast, families like Poaceae (grasses), Solanaceae (nightshades), and Compositae (asters) do not have root nodules and cannot fix atmospheric nitrogen, making them less suitable as primary green manure crops.