The plant part responsible for the formation of fruits is the ovary.

Here's why:

* Flowers: Fruits develop from the flowers of a plant.

* Ovary: The ovary is the female reproductive part of a flower. It contains one or more ovules, which are the structures that will develop into seeds.

* Pollination: When a flower is pollinated, the pollen grain fertilizes the ovule. This triggers the ovary to start developing into a fruit.

So, the ovary is the key part that transforms into the fleshy or dry structure we call a fruit, which serves to protect and disperse the seeds within.