**Core Concept**
The diameter of the Graafian follicle at the time of ovulation is a critical aspect of female reproductive physiology. The Graafian follicle is a mature ovarian follicle that releases an egg (ovum) during ovulation, and its size is an indicator of the approaching ovulatory event.

**Why the Correct Answer is Right**
The Graafian follicle typically reaches a diameter of approximately 18-20 mm at the time of ovulation. This increase in size is due to the accumulation of fluid within the follicle, which leads to a significant expansion of the follicular wall. The larger size of the Graafian follicle is accompanied by an increase in the levels of estrogen produced by the follicle, which helps to prepare the uterus for a potential pregnancy. The increase in estrogen levels also stimulates the growth of the endometrium, preparing it for implantation of a fertilized egg.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the diameter of the Graafian follicle at the time of ovulation is not typically less than 16 mm.
**Option B:** This option is incorrect because the diameter of the Graafian follicle at the time of ovulation is not typically greater than 22 mm.
**Option C:** This option is incorrect because the diameter of the Graafian follicle at the time of ovulation is not typically less than 14 mm.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the diameter of the Graafian follicle can be an indicator of the approaching ovulatory event, and its size can be monitored through ultrasound to predict the timing of ovulation.

**Correct Answer: C. 18-20 mm**