To solve this problem, we need to identify the main function of the mammalian corpus luteum. Let's analyze the options:$\newline \bullet$Option 1: Estrogen only$\newline$ - The corpus luteum does produce estrogen, but it is not its main function.$\newline \bullet$Option 2: Progesterone$\newline$ - The primary function of the corpus luteum is to produce progesterone.$\newline$ - Progesterone is crucial for maintaining the uterine lining and supporting early pregnancy.$\newline \bullet$Option 3: Human chorionic gonadotropin$\newline$ - Human chorionic gonadotropin (hCG) is produced by the placenta, not the corpus luteum.$\newline \bullet$Option 4: Relaxin only$\newline$ - Relaxin is produced by the corpus luteum, but it is not its main function.$\newline$Therefore, the main function of the mammalian corpus luteum is to produce progesterone.$\newline$This corresponds to Option 2.