**Core Concept**
The corpus luteum is a temporary endocrine structure in the female ovary that plays a crucial role in the menstrual cycle and early pregnancy. It forms from the ruptured follicle after ovulation and secretes essential hormones to maintain pregnancy.

**Why the Correct Answer is Right**
The corpus luteum primarily secretes progesterone, which prepares the uterine lining for implantation of a fertilized egg. Progesterone inhibits uterine contractions, promotes endometrial growth, and maintains a favorable environment for embryonic development. Additionally, the corpus luteum secretes small amounts of estrogen, which helps to thicken the endometrium.

**Why Each Wrong Option is Incorrect**
**Option A:** The corpus luteum does not primarily secrete follicle-stimulating hormone (FSH), which is produced by the pituitary gland and regulates follicular growth in the ovaries.
**Option B:** Although the corpus luteum secretes some estrogen, it is not its primary hormone secretion.
**Option C:** The corpus luteum does not secrete luteinizing hormone (LH), which is also produced by the pituitary gland and triggers ovulation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the corpus luteum's primary function is to secrete progesterone, which is critical for maintaining early pregnancy. Any disruption in progesterone production can lead to miscarriage or failure to conceive.

**Correct Answer:** A. Progesterone.