## Core Concept
The menstrual cycle is regulated by a complex interplay of hormones, including estrogen and progesterone. Progesterone plays a crucial role in preparing the uterus for a fertilized egg by thickening the uterine lining. Its levels vary throughout the menstrual cycle.

## Why the Correct Answer is Right
Progesterone levels increase after ovulation, which typically occurs around day 14 of a 28-day menstrual cycle. Following ovulation, the empty follicle in the ovary transforms into the corpus luteum, which secretes progesterone. The peak level of progesterone is usually reached around day 21 of a 28-day menstrual cycle. This peak is critical for preparing the uterine lining for potential implantation of a fertilized egg.

## Why Each Wrong Option is Incorrect
* **Option A:** This option suggests that progesterone peaks early in the cycle, which is not accurate. Progesterone levels are low during the follicular phase.
* **Option B:** This option is incorrect because while ovulation typically occurs around day 14, progesterone levels continue to rise after ovulation, peaking later.
* **Option D:** This option suggests a peak later than expected. While there can be variation in menstrual cycle length, a peak at day 28 is generally considered late for a typical 28-day cycle.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the timing of progesterone peak can help in assessing ovulation and luteal phase adequacy. For a 28-day menstrual cycle, progesterone typically peaks around day 21. This information is crucial in evaluating infertility and managing hormonal imbalances.

## Correct Answer: C. 21.