Each option presents a type of plant part, however, only option 4, the potato, is a stem. Here`s why:

Option 1, Carrot, is a root. It is a type of modified root known as a taproot, which stores energy in the form of carbohydrates.

Option 2, Sweet Potato, is also a type of root, specifically, a tuberous root. This form of root is similar to the carrot, also storing carbohydrates.

Option 3, Coconut, is a fruit. Its function is to aid in the dispersal of seeds. It is neither a root nor a stem.

Option 4, Potato, is a stem modification known as a tuber. A tuber is a short, thick, underground stem that is utilized for storing nutrients. This is why potatoes have "eyes", which are actually buds for new plants.

So, the potato is the correct option as it is a modified stem, while the rest options are either roots or fruit.