The correct answer to the question is option 2: Sweet Potato.

The stem of a plant is generally the part that connects the roots and leaves, and it is responsible for providing support to the plant. In the case of Sweet Potato, the stem is typically found underground.

Option 1: Banana. The stem of a banana plant is not usually found underground. It is an above-ground stem that grows from an underground rhizome.

Option 3: Tapioca. Tapioca is a starch extracted from cassava roots, which are underground tuberous roots. The stem of the cassava plant is not commonly underground.

Option 4: Watermelon. The stem of a watermelon plant is not usually underground. It grows above ground and connects the roots to the leaves and fruits.

In summary, the correct answer is option 2: Sweet Potato, as the stem of a sweet potato plant is typically found underground.