**Core Concept**
Decidualization of the endometrium refers to the transformation of the glandular and stromal cells of the endometrium in preparation for a potential pregnancy. This process involves changes in the morphology, biochemistry, and function of the endometrial cells, making it a crucial step in the establishment of a successful pregnancy.

**Why the Correct Answer is Right**
Progesterone plays a pivotal role in decidualization by binding to the progesterone receptor in endometrial stromal cells. This binding activates a cascade of downstream effects, including the upregulation of enzymes involved in the synthesis of glycogen and the downregulation of enzymes involved in the breakdown of glycogen. Progesterone also promotes the differentiation of endometrial stromal cells into decidual cells, which are characterized by their large size, abundant cytoplasm, and specialized function in supporting embryonic implantation.

**Why Each Wrong Option is Incorrect**
**Option A:** Estrogen is responsible for the proliferation of the endometrium during the follicular phase of the menstrual cycle, but it does not play a direct role in decidualization.

**Option C:** Inhibin is a protein produced by the granulosa cells of the ovary that inhibits the production of follicle-stimulating hormone (FSH). It is not directly involved in decidualization.

**Option D:** FSH is a hormone produced by the pituitary gland that stimulates the growth and maturation of ovarian follicles. While FSH is essential for ovulation, it does not play a direct role in decidualization.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that decidualization is a progesterone-dependent process. In cases of luteal phase defect or progesterone deficiency, decidualization may be impaired, leading to implantation failure or early pregnancy loss.

**✓ Correct Answer: B. Progesterone. Progesterone plays a crucial role in decidualization of the endometrium.**