The ovary of the flower develops into a fruit.
Once the flower has been polinated, it eventually turns into a fruit. This happens in the female part of the flower.
ovary
seed
flower
The ovary
anther
the ovary
the ovary
gymonecium
The part of a plant that changes into a fruit is the Ovule (the female section of the flower).
In most plants, this would be the flower; specifically, the petals.