**Core Concept**
The release of an ovum, also known as ovulation, is a critical event in the female reproductive cycle. It is primarily controlled by the hypothalamic-pituitary-gonadal axis, with the pituitary gland releasing luteinizing hormone (LH) that triggers ovulation. This process involves the rupture of the dominant follicle in the ovary, releasing a mature ovum into the peritoneal cavity.

**Why the Correct Answer is Right**
The correct answer is **C. Luteinizing hormone (LH)**. LH surge triggers the ovulation process by binding to its receptors on the surface of the granulosa cells of the dominant follicle, causing a rapid increase in the production of progesterone and a weakening of the follicular wall. This leads to the rupture of the follicle and the release of the ovum into the peritoneal cavity. The LH surge is triggered by a decrease in estrogen levels, which is sensed by the hypothalamus and results in the release of gonadotropin-releasing hormone (GnRH).

**Why Each Wrong Option is Incorrect**
**Option A:** Follicle-stimulating hormone (FSH) is responsible for the growth and development of follicles in the ovary, but it does not trigger ovulation.

**Option B:** Estrogen is involved in the regulation of the menstrual cycle, but it does not directly trigger ovulation. A decrease in estrogen levels actually triggers the LH surge.

**Option D:** Progesterone is involved in the preparation of the uterus for implantation of a fertilized ovum, but it does not trigger ovulation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ovulation is a critical event in the female reproductive cycle, and its timing is regulated by the hypothalamic-pituitary-gonadal axis. A decrease in estrogen levels triggers the LH surge, which in turn triggers ovulation.

**Correct Answer: C. Luteinizing hormone (LH)**