Correct Option: 2
Step 1: Function of theca interna. The theca interna, a layer within the ovarian follicle, is essential for hormone synthesis. It produces androgens, which serve as estrogen precursors.
Step 2: Hormone production. Granulosa cells, stimulated by FSH, transform the androgens from the theca interna into estrogen via aromatization.
Step 3: Analysis of alternative options. Option (A): Incorrect. Progesterone production is mainly attributed to the corpus luteum, not the theca interna.
Option (C): Incorrect. LH stimulates the theca interna but is not produced by it.
Option (D): Incorrect. FSH influences granulosa cells, not the theca interna. \[ \therefore \text{The correct answer is: Estrogen.} \]