**Core Concept**
The spinbarkeit, also known as the "egg-white" appearance, of cervical mucus refers to its ability to be stretched and forms a long thread. This characteristic is crucial in fertility assessment and is influenced by estrogen levels in the menstrual cycle.

**Why the Correct Answer is Right**
Estrogen causes the cervical mucus to become more watery, clear, and abundant, with increased spinbarkeit. This is due to the stimulation of the enzyme glycogen synthase, which increases the production of glycogen. Glycogen is then converted to glucose by the enzyme glycogen phosphorylase, which attracts and retains sperm. The peak estrogen levels occur around the ovulation time, making the cervical mucus optimal for sperm penetration and fertility.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because estrogen levels are not typically associated with the premenstrual phase of the menstrual cycle, where progesterone dominates and cervical mucus becomes cloudy and thick.
* **Option B:** This option is incorrect because follicular phase, which occurs before ovulation, has increasing estrogen levels, but the peak estrogen levels occur around ovulation time, making option C more accurate.
* **Option D:** This option is incorrect because luteal phase, which occurs after ovulation, has dominant progesterone levels, leading to thick and cloudy cervical mucus, making it less optimal for sperm penetration.

**Clinical Pearl / High-Yield Fact**
The ferning pattern of cervical mucus, visible under a microscope, can also be used to assess fertility. The ferning pattern is most pronounced during the fertile window, around ovulation time.

**Correct Answer:** C. Fertile window, around ovulation time.