**Core Concept**
The Graafian follicle is a mature ovarian follicle that is ready to release an ovum. It is a critical structure in the menstrual cycle, playing a key role in ovulation and subsequent fertilization. The size of the Graafian follicle is an important indicator of its readiness to release the ovum.

**Why the Correct Answer is Right**
The Graafian follicle typically reaches a diameter of 18-20 mm before ovulation. This increase in size is due to the accumulation of fluid within the follicle, which is essential for ovum release. The follicular cells undergo significant changes, including the formation of a fluid-filled antrum, to facilitate ovulation. The increased size and fluid content of the Graafian follicle are critical for successful ovulation and subsequent fertilization.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the size of the Graafian follicle before ovulation is typically larger than 14 mm.
**Option B:** This option is incorrect as the Graafian follicle does not reach a diameter of 24 mm before ovulation.
**Option C:** This option is incorrect as it does not provide a specific diameter for the Graafian follicle before ovulation.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the size of the Graafian follicle can be assessed through ultrasound, which can help predict ovulation and fertility. A diameter of 18-20 mm is a critical threshold for ovulation, and its measurement can aid in the management of fertility issues.

**Correct Answer:** C. 20 mm