**Core Concept**
The corpus luteum is a temporary endocrine structure in the female ovaries that plays a crucial role in early pregnancy by producing progesterone. Its function is vital for maintaining the endometrium and preparing it for implantation of a fertilized egg.

**Why the Correct Answer is Right**
The corpus luteum's activity is maintained by human chorionic gonadotropin (hCG) produced by the placenta after implantation of the embryo. hCG has a similar structure to luteinizing hormone (LH) and binds to the LH receptor on the corpus luteum, stimulating it to continue producing progesterone. This ensures the maintenance of the endometrium and supports embryonic development during the early stages of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the maintenance of corpus luteum activity.
**Option B:** While estrogen is an important hormone in the female reproductive cycle, it is not responsible for maintaining corpus luteum activity.
**Option C:** Progesterone is a product of the corpus luteum, not a factor that maintains its activity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the corpus luteum's function is crucial for early pregnancy, and any disruption in its activity can lead to miscarriage. The role of hCG in maintaining corpus luteum activity is a key concept in reproductive endocrinology.

**Correct Answer:** C. Human placental lactogen (hPL)