**Core Concept**
FSH (Follicle-Stimulating Hormone) secretion is regulated by a complex interplay of hormones and neural signals in the hypothalamic-pituitary-gonadal axis. FSH stimulates the growth and maturation of ovarian follicles in females and spermatogenesis in males.

**Why the Correct Answer is Right**
FSH secretion is primarily stimulated by the hypothalamic hormone **Gonadotropin-Releasing Hormone (GnRH)**. GnRH stimulates the release of FSH and LH (Luteinizing Hormone) from the anterior pituitary gland. This GnRH-FSH axis is essential for reproductive function in both males and females. In females, GnRH stimulates the release of FSH, which in turn promotes the growth of ovarian follicles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Adrenaline (Epinephrine)** is a stress hormone that has various effects on the body, but it does not directly stimulate FSH secretion.
**Option B:** This option is incorrect because **Prolactin** is a hormone involved in lactation and has a suppressive effect on GnRH and FSH secretion.
**Option C:** This option is incorrect because **Insulin** is a hormone involved in glucose metabolism and does not have a direct effect on FSH secretion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the hypothalamic-pituitary-gonadal axis is a critical regulator of reproductive function. GnRH analogs, which mimic or inhibit GnRH, are used in the treatment of various reproductive disorders, including precocious puberty and infertility.

**Correct Answer:** D. GnRH.