**Core Concept**
Progesterone is a key steroid hormone produced in the ovary during the luteal phase of the menstrual cycle, primarily by the corpus luteum, which develops from the remnants of the follicle after ovulation.

**Why the Correct Answer is Right**
After ovulation, the remnants of the follicle transform into the **corpora lutea**, which are highly active in secreting **progesterone** and estrogen. This hormone prepares the endometrium for potential implantation and maintains the uterine lining. The process involves the conversion of androgens (like androstenedione) to progesterone via **3β-hydroxysteroid dehydrogenase** and **17α-hydroxylase** enzymes in the granulosa and theca cells of the follicle, but progesterone production is dominant in the corpus luteum. The corpus luteum is the primary source of progesterone in the post-ovulatory phase.

**Why Each Wrong Option is Incorrect**
Option A: The **stroma** is the connective tissue of the ovary and produces minimal steroid hormones; it is not the primary source of progesterone.
Option B: **Corpora albicans** are fibrous remnants of the corpus luteum after it regresses, and they do not produce progesterone.
Option D: **Mature follicles** secrete small amounts of estrogen and some progesterone before ovulation, but their progesterone output is negligible compared to the corpus luteum.

**Clinical Pearl / High-Yield Fact**
Remember: *The corpus luteum is the main progesterone producer in the ovary — it forms after ovulation and regresses if pregnancy does not occur.*

✓ Correct Answer: C. Corpora lutea