**Core Concept**
During the first trimester of pregnancy, the corpus luteum plays a crucial role in maintaining progesterone levels, which are essential for the continued growth and development of the embryo. The corpus luteum is a temporary endocrine structure that forms in the ovary after ovulation and secretes progesterone to support the early stages of pregnancy.

**Why the Correct Answer is Right**
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta that stimulates the corpus luteum to continue producing progesterone during the first trimester. hCG binds to the luteinizing hormone (LH) receptor on the corpus luteum, mimicking the action of LH and preventing the corpus luteum from degenerating. This ensures the continued production of progesterone, which is essential for the maintenance of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while progesterone is essential for pregnancy, it is not the hormone responsible for maintaining the corpus luteum during the first trimester.
**Option B:** This option is incorrect because human placental lactogen (hPL) is a hormone produced by the placenta, but it does not play a role in maintaining the corpus luteum.
**Option C:** This option is incorrect because relaxin is a hormone produced by the placenta and corpus luteum, but it is not responsible for maintaining the corpus luteum during the first trimester.

**Clinical Pearl / High-Yield Fact**
The maintenance of the corpus luteum by hCG is a critical process that supports the early stages of pregnancy. Failure of this process can lead to miscarriage or other pregnancy complications.

**Correct Answer:** hCG. Human chorionic gonadotropin.