**Core Concept**
The ferning of cervical mucus is a phenomenon where the mucus takes on a fern-like pattern under microscopic examination. This occurs due to the crystallization of the mucus when it dries, and its disappearance is an indicator of the menstrual cycle's progression.

**Why the Correct Answer is Right**
Ferning of cervical mucus is typically observed in the proliferative phase of the menstrual cycle, up to the 14th day. The disappearance of ferning is due to the shift from a more alkaline to a more acidic cervical mucus environment, which occurs after ovulation. This shift is caused by the increase in estrogen levels before ovulation and the subsequent decrease in estrogen levels after ovulation, which leads to an increase in progesterone levels. Progesterone causes the cervical mucus to become more alkaline and less conducive to ferning.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ferning can be observed in the early proliferative phase, but not as late as the 21st day of the menstrual cycle.
**Option B:** This option is incorrect because the disappearance of ferning is not related to the onset of menstruation, but rather to the shift in hormonal balance after ovulation.
**Option C:** This option is incorrect because ferning is not directly related to the presence or absence of ovulation, but rather to the hormonal changes that occur after ovulation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the disappearance of ferning is a reliable indicator of ovulation and can be used in conjunction with other clinical and laboratory parameters to diagnose ovulatory disorders.

**Correct Answer: D. 21st day**