The correct option is
B Stem
Sweet potato is a modified root, whereas potato is modified stem. Modified stems are derived from stem tissue that exists under the soil surface. Various modified stems are tuber, bulb, corm, stolon and rhizome. Tuber is a thickened part of a stem which grows underground. Tubers have bud from which a new plant can arise by vegetative propagation. Potato is a modified stem called a tuber. It stores a large amount of edible starch which is used as food.Thus, the correct answer is option B.