The correct answer for this question is option 1, tuber. A tuber is a type of stem modification that stores nutrients for the plant. It is an underground, swollen structure that forms from an enlarged portion of a stem called a stolon. This stolon grows horizontally underground and gives rise to the tuber. The potato is a tuber because it grows underground and is swollen with starch, which serves as a nutrient reserve for the plant.

Option 2, bulb, is incorrect. A bulb is a modified underground bud with fleshy storage leaves surrounding it. Examples of bulbs include onions and tulips.

Option 3, corms, is also incorrect. Corms are thick, vertical underground stems that store nutrients. Examples of corms include gladiolus and crocus.

Option 4, rhizome, is also incorrect. Rhizomes are horizontal underground stems that grow parallel to the soil surface. Examples of rhizomes include ginger and bamboo.

Therefore, the correct answer is tuber.