The correct answer is B: a fertilized ovary.

A true fruit is formed from the fertilized ovary of a flower. After fertilization, the ovary develops into a fruit, and the ovules within the ovary become seeds.

Here’s why the other options are incorrect:
- A: A true fruit is formed from the fertilized ovary, not other floral parts. Other parts like the petals or sepals may be involved in attracting pollinators but do not contribute to the formation of the fruit.
- C: The calyx (the sepals) typically does not contribute to the formation of a true fruit.
- D: The fleshy receptacle is part of some fruits (like in apples or strawberries), but it is not essential for a true fruit. The fertilized ovary is what primarily forms the fruit.
- E: An unfertilized ovary does not lead to a true fruit. A fruit develops only after fertilization.