The correct answer is option 3 - ovary. In botany, a fruit is the mature ovary of a flowering plant. The ovary is the part of the flower that contains the ovules, which eventually develop into seeds after fertilization. When the ovules are fertilized, the ovary begins to develop into a fruit.

Option 1 - endosperm, is incorrect as endosperm is a tissue in the seed that provides nutrients to the developing embryo and is not directly related to fruit development.

Option 2 - style and stigma, are parts of the female reproductive organs of a flower, but they are not the main contributors to fruit development. The style is a slender tube connecting the stigma to the ovary, and the stigma is the part of the flower that receives the pollen. However, they do not transform into the fruit.

Option 4 - flower stalk, is incorrect as the flower stalk, also known as the peduncle, supports the entire flower but does not play a direct role in fruit development.

To summarize, in a flowering plant, the ovary undergoes transformation into a fruit after fertilization, making option 3 - ovary, the correct answer.