Explanation:
The correct answer is C: pistil (gynoecium).

The pistil, also known as the gynoecium, is the female reproductive part of the flower. It consists of the stigma, style, and ovary. For pollination and fruit formation to occur, the pistil plays a crucial role:

- Pollination occurs when pollen from the male part of the flower (anther) reaches the stigma of the pistil.
- After successful pollination, the ovary (part of the pistil) contains the ovules, which develop into seeds, and the ovary itself develops into a fruit.

Here's why the other options are incorrect:
- A. Corolla: The corolla is the collective term for the petals of a flower, which attract pollinators but are not directly involved in the formation of fruit.
- B. Ovary: The ovary is a part of the pistil (gynoecium), and while it is essential for fruit formation, the pistil as a whole is necessary for pollination and fruit formation.
- D. Ovules: The ovules are located within the ovary and are the structures that develop into seeds, but they cannot form fruit without pollination and fertilization, which involves the entire pistil.
- E. Receptacle: The receptacle is the part of the flower stalk that holds the flower, but it does not directly participate in pollination or fruit formation.

Therefore, the pistil (gynoecium) is the essential part of the flower for pollination and fruit formation.