The correct answer is option 2: Stem. Potatoes are actually modified stems called tubers. Although they grow underground like roots, they are not roots. Potatoes grow from the lower portion of the stem, called the stolon, which produces swollen structures that store nutrients for the plant. These swollen structures are what we commonly refer to as potatoes.

Option 1, Root, is incorrect because potatoes are not roots.

Option 3, Bud, is also incorrect. While potatoes do have buds called "eyes," these are not the main characteristic of a potato.

Option 4, Fruit, is incorrect because potatoes are not fruits. Fruits are typically the mature ovaries of flowering plants, and potatoes do not fit this description.

In summary, potatoes are modified stems called tubers that grow underground and store nutrients for the plant.