**Core Concept**
The ovarian cycle is a complex, tightly regulated process that involves the coordinated action of multiple hormones and the growth and development of ovarian follicles. The initiation of the ovarian cycle is a critical step that sets the stage for ovulation and the subsequent luteal phase.

**Why the Correct Answer is Right**
The ovarian cycle is initiated by the surge of Follicle-Stimulating Hormone (FSH) from the anterior pituitary gland. FSH acts on the granulosa cells of the growing ovarian follicles, stimulating them to produce estrogen and promote follicular growth. The increase in estrogen levels in turn feeds back to the pituitary gland, leading to a surge in Luteinizing Hormone (LH) that triggers ovulation. However, the initial trigger for this process is the release of FSH.

**Why Each Wrong Option is Incorrect**
**Option B:** Estrogen plays a crucial role in the ovarian cycle, but it is not the initiator. Instead, estrogen levels rise in response to FSH stimulation.
**Option C:** LH is essential for ovulation, but it is not the initial trigger for the ovarian cycle. The LH surge is a response to the rising estrogen levels triggered by FSH.
**Option D:** Progesterone is produced by the corpus luteum after ovulation and is essential for preparing the uterus for potential implantation of a fertilized egg. However, it is not involved in the initiation of the ovarian cycle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the ovarian cycle is a tightly regulated process that involves the interplay of multiple hormones. FSH and LH are the key players, with FSH initiating the process and LH triggering ovulation. This understanding is crucial for managing conditions such as polycystic ovary syndrome (PCOS) and infertility.

**✓ Correct Answer: A. FSH**