Question

potato is stem and sweet potato is a root

Answer 1

Sweet potato is a modified food storage root, of fibrous roots. Yes potato is a stem which grows on underground stems which are called stolons. Potato tubers have buds that sprout leaves and stems from them while roots do not posses these properties. While sweet potato is root filled with food.
I guess....

Answer 2

Potato is a stem and sweet potato is a root.

a)Potato are stem tubers because they are fleshy,swollen, rounded or oblong distal portions of underground axillary or adventitious branches.These branches arise from the underground basal nodes of the aerial shoots. The tips of these branches become enlarged in the form of tubers, due to accumulation of surplus food material manufactured by the aerial shoots.

(b) Sweet potato is a root tuber because these are adventitious roots that become thick and fleshy due to storage of food and are swollen without any definite shape. The root tubers of sweet potato also bear adventitious buds from which new plants can develop.

hope it helps u :)