**Core Concept**
Inhibin is a protein hormone that plays a crucial role in regulating the reproductive cycle by modulating the secretion of follicle-stimulating hormone (FSH) from the anterior pituitary gland. Inhibin is produced by the granulosa cells of the ovaries in females and the Sertoli cells of the testes in males.

**Why the Correct Answer is Right**
Inhibin acts on the anterior pituitary gland to inhibit the secretion of FSH through a negative feedback mechanism. This is essential for maintaining a delicate balance in the reproductive cycle, as excessive FSH can lead to over-stimulation of follicular growth and development, potentially resulting in ovarian cysts or tumors. Inhibin binds to its receptor on the surface of pituitary cells, triggering a signaling cascade that ultimately suppresses the production and release of FSH.

**Why Each Wrong Option is Incorrect**
**Option A:** While estrogen is also a hormone involved in regulating the reproductive cycle, it primarily stimulates FSH secretion, not inhibits it.
**Option B:** Testosterone is a sex hormone produced by the testes in males, but it does not directly inhibit FSH secretion. Instead, it is involved in regulating spermatogenesis and secondary sex characteristics.
**Option C:** Progesterone is a hormone produced by the corpus luteum in the ovaries, but it does not directly inhibit FSH secretion. Instead, it prepares the uterus for implantation of a fertilized egg.

**Clinical Pearl / High-Yield Fact**
Inhibin is a critical regulatory hormone that helps maintain a balance between FSH and estrogen levels in the reproductive cycle. An imbalance in inhibin levels can lead to disorders such as polycystic ovary syndrome (PCOS) or premature ovarian failure (POF).

**Correct Answer:** A. Inhibin.