The correct answer is option 4: Fruit. In plants, the mature ovary undergoes changes and develops into a fruit. The fruit is the reproductive structure of the plant that contains seeds. It plays a crucial role in seed dispersal as it protects the seeds and aids in their transportation.

Option 1, Seed, is a reproductive structure that is typically found within the fruit. It is derived from the fertilized ovule and contains the embryo, which has the potential to grow into a new plant.

Options 2 and 3, Stamen and Pistil, are reproductive structures involved in the process of pollination and fertilization. The stamen is the male reproductive part, while the pistil is the female reproductive part. However, neither the stamen nor the pistil undergoes a change into the mature ovary.