**Core Concept**
Apoptosis, or programmed cell death, is a regulated process that eliminates damaged or unwanted cells in multicellular organisms. In the context of the ovarian cycle, apoptosis plays a crucial role in maintaining endometrial homeostasis.

**Why the Correct Answer is Right**
When there is no fertilization of the ovum, the corpus luteum degenerates, leading to a decrease in progesterone levels. This decrease in progesterone triggers a cascade of molecular events that ultimately result in the induction of apoptosis in endometrial cells. The downregulation of progesterone receptors and the upregulation of pro-apoptotic genes, such as BAX, contribute to the initiation of apoptosis. The involvement of caspases, particularly caspase-3, in the execution of apoptosis further ensures the precise elimination of endometrial cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the molecular mechanism underlying apoptosis in endometrial cells. While changes in hormone levels are involved, the specific role of progesterone withdrawal is crucial.

**Option B:** This option is incorrect because it is too vague and does not specify the underlying hormonal changes that trigger apoptosis.

**Option C:** This option is incorrect because it refers to a different process, where the endometrium is shed as part of menstruation, but does not accurately describe the molecular mechanism of apoptosis.

**Clinical Pearl / High-Yield Fact**
The withdrawal of progesterone is a key trigger for apoptosis in endometrial cells, and understanding this mechanism is essential for grasping the complex interplay between hormones and cellular regulation in the female reproductive system.

**Correct Answer:** C.