Option 1: Potato -

Potatoes are actually tubers that grow underground, not fruits. Therefore, this option is incorrect.

Option 2: Carrot -

Carrots are root vegetables, not fruits. They grow underground, but they do not bear fruits. Hence, this option is also incorrect.

Option 3: Groundnut -

Groundnuts, also known as peanuts, are legumes that grow underground. While they may have a fruit-like structure called a pod, they are not technically fruits. However, this option is correct as it is the only option that mentions a plant whose edible part is found underground.

Option 4: Onion -

Onions are bulb vegetables that grow underground. They do not produce any fruits. Therefore, this option is incorrect.

In conclusion, the correct answer is option 3 - Groundnut. The fruits of the groundnut plant are found underground in the form of pods, which contain the edible groundnuts.