Geocarpic fruit is : -

Geocarpy is the development of fruits underground.
In peanut (groundnut, Arachis hypogaea), after fertilisation the stalk of the ovary (gynophore) elongates, bends down and pushes the young ovary into the soil, where the pod develops and ripens.
Potato is a stem tuber and onion and garlic are bulbs; these are underground stems, not fruits.
So the geocarpic fruit is peanut.