The potatoes are called modified stems because of

Concept: Potato is an underground modified stem, though It stores reserved food. Potato is a tuber. It has scaly leaves, internodes, nodes, and adventitious roots, these all are characteristics of a stem. Potato also has buds.
The eye of the potato develops into a new seedling from which a new potato can grow. Many people think a potato is a root as it grows inside the soil, but a potato is a modified stem that stores food in them. Potato is also known as the tuberous stem. Tuber mainly occurs in an oval, round and cylindrical shape.
Hence, the correct answer is B.