Answer:
A potato isn't a fruit. The part of the potato we eat is its edible tuber, which is classed as a vegetable.
A vegetable is the edible part of a plant that is not a fruit or seed, and although this is not a strict definition, it is safe to say a potato is a vegetable.
The potato produces stem tubers, a development of the normal stem. It's perhaps the only stem tuber that we eat. Root tubers are more common and include sweet potato and cassava. These tubers are distinct from bulbs and corms.
The potato plant produces a fruit, but this is not what we eat; itsfruit is poisonous, as are many parts of plants which, like potatoes, are members of the Solanaceae - nightshade - family: for example, only the fruit of the tomato, bell pepper or chili plant is edible. In the case of potatoes we eat the tuber rather than the fruit.