Which of following plant is used as green manure?

Vigna sinensis or cowpea is one of the most widely used legumes in the tropical world. The grain is used widely for human nutrition, especially in Africa. It is one of the most important tropical dual-purpose legumes, being used for vegetables (leaves and flowers), grain, as fresh cut and carry forage, and for hay and silage. The species has a high potential as a green manure. It can be incorporated into the soil or spread on the soil surface 8-10 weeks after sowing and can provide the equivalent of 80 kg/ha N to a subsequent crop.  

Berseem is a nitrogen-fixing legume. It may require Rhizobium inoculation outside its native area. As a winter crop, it provides soil cover and prevents erosion. 

So, the correct answer is option D.