Which of the following plant hormones stimulates the development of parthenocarpic fruit?

The correct option is All of the above.

Parthenocarpy is the process of fruit development without prior fertilization, which results in seedless fruits. Plant hormones (phytohormones) play a crucial role in regulating this process. Let us look at how each of the listed hormones contributes to the induction of parthenocarpic fruit development:

1. Auxins: Auxin is one of the primary hormones responsible for fruit growth. High levels of auxin in the ovary stimulate cell division and elongation, mimicking the hormonal trigger normally provided by pollination and fertilization. Applying exogenous auxin to unpollinated flowers commonly induces parthenocarpy in plants like tomatoes.

2. Gibberellins: Gibberellins also stimulate cell division and elongation. They are highly effective in inducing seedless fruit development, particularly in fruits like grapes, apples, and stone fruits, where they can bypass the requirement of pollination.

3. Cytokinins: Cytokinins promote cell division (cytokinesis) and work in synergy with auxins to promote ovary growth and fruit set even in the absence of fertilization. Their application has been shown to induce parthenocarpy in various plant species.

Because Auxins, Cytokinins, and Gibberellins are all capable of stimulating the development of parthenocarpic fruits, the correct answer is indeed "All of the above".