**Core Concept**
The luteal phase of the menstrual cycle is characterized by the production of progesterone by the corpus luteum, which prepares the uterus for a potential pregnancy. This phase is crucial for maintaining the endometrium and supporting embryonic development.

**Why the Correct Answer is Right**
Progesterone levels peak during the luteal phase, approximately 7-10 days after ovulation. This hormone is essential for preparing the uterus for implantation by inducing endometrial proliferation, differentiation, and secretion. The increase in progesterone levels also promotes the maintenance of the endometrial lining, which is necessary for a successful pregnancy. The corpus luteum, formed from the ruptured follicle, produces progesterone through the action of enzymes such as cholesterol desmolase and 3β-hydroxysteroid dehydrogenase.

**Why Each Wrong Option is Incorrect**
* **Option A:** Estrogen levels are highest during the follicular phase, particularly during the mid-cycle surge, which triggers ovulation.
* **Option B:** Human chorionic gonadotropin (hCG) is produced by the placenta during pregnancy, not during the luteal phase.
* **Option D:** Progesterone is not the highest during the follicular phase; estrogen is the dominant hormone during this time.

**Clinical Pearl / High-Yield Fact**
It is essential to note that progesterone-only pills (POPs) and progesterone-releasing intrauterine devices (IUDs) are used as contraceptive methods that work by disrupting the endometrial lining, making it unfavorable for implantation.

**Correct Answer:** C. Progesterone.