## Core Concept
Inhibins are proteins that inhibit the production of follicle-stimulating hormone (FSH) by the anterior pituitary gland. They are part of the TGF-β (transforming growth factor-beta) superfamily and play a crucial role in regulating reproductive functions.

## Why the Correct Answer is Right
The correct answer, **inhibin**, is secreted by **Graafian follicles (or antral follicles)**, which are mature ovarian follicles. These follicles produce inhibin, which then acts on the anterior pituitary gland to decrease FSH secretion. This feedback mechanism is essential for the proper regulation of the menstrual cycle and fertility.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately represent the source of inhibin secretion.
* **Option B:** This option is incorrect as it does not correctly identify the structure responsible for inhibin secretion.
* **Option C:** This option might seem plausible but is not the correct source of inhibin secretion.

## Clinical Pearl / High-Yield Fact
A key point to remember is that inhibins are dimeric proteins, consisting of an alpha subunit and one of two beta subunits (βA or βB), leading to the formation of inhibin A (αβA) or inhibin B (αβB). Inhibin B is primarily produced by the granulosa cells of small antral follicles and is a marker of ovarian reserve.

**Correct Answer: C. Graafian follicles.**