The correct answer is option 4, Potato. A modified underground stem refers to a stem that grows and stores food underground. Out of all the options given, only the potato fits this description. Potatoes are tubers, which are swollen, underground stems that store starch and other nutrients for the plant. These modified stems are capable of producing new shoots and roots if they are planted.

Option 1, Radish, and option 2, Carrot, are both examples of modified underground roots. They store food for the plant and do not produce new roots or shoots. Option 3, Sweet Potato, is also a modified underground root, similar to the carrot and radish.

In conclusion, when looking for a modified underground stem, the correct option is potato.