**Core Concept**
The mature Graafian follicle is a crucial structure in the female reproductive system, playing a key role in ovulation. It is the final stage of follicular development before ovulation, characterized by a large fluid-filled cavity. The size of the Graafian follicle is an important indicator of its maturity and readiness for ovulation.

**Why the Correct Answer is Right**
The preovulatory Graafian follicle typically reaches a diameter of approximately 18-24 mm before ovulation. This size increase is due to the accumulation of fluid within the follicle, known as follicular fluid, which helps to mature the oocyte and prepare it for release. The correct answer reflects the typical size threshold below which a mature Graafian follicle is considered preovulatory.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the size of a preovulatory Graafian follicle.
**Option B:** This option is also incorrect as it exceeds the typical size range for a preovulatory follicle.
**Option C:** Similarly, this option is incorrect as it is not consistent with the expected size of a mature Graafian follicle before ovulation.
**Option D:** This option is incorrect because it is above the typical size threshold for a preovulatory follicle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the size of the Graafian follicle is a critical factor in determining the timing of ovulation. A follicle that reaches a diameter of 18-24 mm is considered mature and ready for ovulation.

**Correct Answer:** Correct Answer: D. 18mm