**Core Concept**
The menstrual cycle is a complex hormonal process regulated by the hypothalamic-pituitary-ovarian axis. Progesterone is a crucial hormone involved in preparing the uterus for pregnancy. During the normal menstrual cycle, progesterone plays a vital role in maintaining pregnancy and supporting embryonic development.

**Why the Correct Answer is Right**
Progesterone is primarily produced by the corpus luteum, a temporary endocrine structure that forms in the ovary after ovulation. The corpus luteum secretes progesterone, which maintains the endometrium's secretory transformation, preparing it for implantation of a fertilized egg. If pregnancy occurs, the placenta takes over progesterone production, ensuring continued support for embryonic growth.

**Why Each Wrong Option is Incorrect**
**Option A:** **Follicular fluid** contains estrogen and other hormones but is not a primary source of progesterone.
**Option B:** **Ovarian stroma** produces some hormones, but it is not the primary source of progesterone during the menstrual cycle.
**Option C:** **Placenta** takes over progesterone production during pregnancy, but it is not the source during the normal menstrual cycle.

**Clinical Pearl / High-Yield Fact**
In the absence of pregnancy, the corpus luteum degenerates, leading to a drop in progesterone levels, which triggers menstruation.

**Correct Answer:** C. Corpus luteum.