Potato is

Botanists distinguish a stem from a root mainly by the presence of nodes (points that bear buds or leaves) and internodes: a stem shows this segmented node-internode pattern and typically carries axillary buds capable of growing into new shoots. A root, by contrast, does not show this node-internode segmentation, and it normally branches only by producing further roots from internal (endogenous) tissue rather than from surface buds.

The potato develops at the swollen tip of an underground horizontal branch called a stolon, which is itself a modified stem growing out from the main stem below the soil surface. The small depressions on the tuber's surface, popularly called 'eyes', are dormant axillary buds sitting exactly at nodes, with the faint lines between them marking internodes -- which is why a piece of tuber carrying an eye can sprout an entirely new plant when planted.

Contrast with the near-miss options:

• Root (e.g. carrot, radish): stores food too, but does not show the node-internode segmentation of a stem, and normally cannot regenerate a full new shoot system directly from its own body.

• Flower: the plant's reproductive organ, produced above ground on a flowering stalk -- it plays no role in underground food storage.

• Leaf: a thin, usually green, photosynthetic organ attached to a stem at a node -- it does not itself swell into a storage organ or sprout a whole new plant.

Because the tuber shows the defining node-internode-bud structure of a stem, the potato is correctly classified as Stem -- specifically, a modified underground stem called a tuber.