**Core Concept**
The cervical mucus at the time of ovulation undergoes a significant change, becoming more watery and favorable for sperm penetration. This is due to the increase in estrogen levels, which stimulates the production of glycoproteins and glycolipids in the cervical mucus, altering its viscosity and texture.

**Why the Correct Answer is Right**
The correct answer is due to the increase in estrogen levels stimulating the production of glycoproteins and glycolipids in the cervical mucus. Estrogen binds to specific receptors in the cervical epithelial cells, leading to the synthesis and secretion of these glycoproteins and glycolipids. These molecules increase the water-binding capacity of the cervical mucus, making it more watery and conducive to sperm penetration.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the change in cervical mucus texture at ovulation.
* **Option B:** Progesterone does not stimulate the production of glycoproteins and glycolipids in the cervical mucus, and its levels actually increase after ovulation, leading to a more viscous and sperm-repellent mucus.
* **Option D:** Testosterone does not play a direct role in the change in cervical mucus texture at ovulation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the estrogen-primed cervical mucus is a critical factor in facilitating sperm penetration and fertilization. This concept is crucial in understanding the female reproductive cycle and fertility.

**Correct Answer: C. Estrogen. Increased estrogen levels stimulate the production of glycoproteins and glycolipids in the cervical mucus, making it more watery and favorable for sperm penetration.**