Among the options given, the crop that is classified as a tuber is c) Cassava. To clarify, tubers are swollen portions of underground stems or roots that serve as storage organs for nutrients.
Onion is actually a bulb, which is the underground storage organ of a plant formed from layers of fleshy leaves. Ginger is a rhizome, a type of underground stem that also serves as a storage unit, but it is not classified as a tuber. Lastly, beans are seeds harvested from the pods of various plants, not tubers.
Therefore, cassava is the correct answer because it is a starchy tuber that is cultivated extensively in tropical regions and is a significant source of carbohydrates.