The correct answer is option 4, which is "stem."

A potato is a type of tuber, which is a specialized stem that grows underground. The edible part of a potato is the swollen portion of the underground stem, known as the tuber. This tuber is rich in starch and nutrients and is what we eat when we consume potatoes. It serves as a food storage organ for the plant and contains essential carbohydrates that provide energy.

Option 1, "bud," is not correct because while a potato may produce buds or shoots, these are not the edible part of the plant.

Option 2, "fruit," is also incorrect because potatoes do not produce fruit. They are not reproductive structures like fruits are in other plants.

Option 3, "root," is not the correct answer either. Although the potato grows underground, it is not a root. It is important to differentiate between roots and stems, as they have different functions and structures.

In conclusion, the correct answer is option 4, "stem," as the edible part of a potato is its underground stem, known as the tuber.