The correct answer is (c) Sunhemp.
Green manure is a type of organic fertilizer that is made from growing plants and then plowing them under before they flower. This helps to improve soil fertility by adding organic matter, nitrogen, and other nutrients. Sunhemp is a particularly good choice for green manure because it grows quickly and has a high nitrogen content. It is also a deep-rooted plant that helps to loosen the soil and improve drainage.
Potato (a) is not a good choice for green manure because it is a root crop that takes up a lot of nutrients from the soil. Sugarcane (b) is also not a good choice because it is a perennial plant that takes several years to mature. Gram (d) is a legume that does fix nitrogen, but it is not as fast-growing or as high in nitrogen as sunhemp.
In conclusion, sunhemp is the most suitable crop for green manuring because it is a fast-growing, high-nitrogen plant that helps to improve soil fertility.