True fruits develop from

A true fruit (eu-fruit) is a structure that develops solely from the ovary of a flower, usually following fertilization: the ovary wall matures into the pericarp that encloses the seed(s) (a few true fruits, called parthenocarpic fruits, form from the ovary even without fertilization, e.g. seedless banana). A fruit that instead forms mainly from an accessory floral part — the thalamus (receptacle), the calyx (sepals), or the corolla (petals) — is classified as a false fruit (pseudocarp), not a true fruit.

Among the parts listed in this question, only the ovary is the female reproductive tissue whose own development gives rise to the fruit body enclosing the seeds, so “ovary” is the correct answer.

• thalamus: the receptacle can swell after fertilization to form the fleshy edible part of accessory fruits such as apple and cashew, but that swelling is receptacle tissue, not ovary tissue — so a thalamus-based fruit is a false fruit.

• petal: petals are the non-reproductive, pollinator-attracting whorl; they typically wither and fall away once fertilization occurs rather than persisting to form fruit tissue.

• sepal: sepals (the calyx) protect the bud before flowering and may occasionally persist around a mature fruit, but the fruit body itself is not built from sepal tissue.

Hence, the ovary is the floral part whose post-fertilization maturation gives rise to a true fruit.