To solve this problem, we need to understand the role of gonadotropins during pregnancy. Let's analyze each option:$\newline \bullet$Option 1: High level of FSH and LH stimulates the thickening of endometrium.$\newline$ - FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone) are primarily involved in the regulation of the menstrual cycle and ovulation.$\newline$ - During pregnancy, the levels of FSH and LH are low because their primary role is not needed.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: High level of FSH and LH facilitate implantation of the embryo.$\newline$ - As mentioned, FSH and LH levels are low during pregnancy.$\newline$ - Implantation is facilitated by the hormone progesterone, not FSH or LH.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 3: High level of hCG stimulates the synthesis of estrogen and progesterone.$\newline$ - Human Chorionic Gonadotropin (hCG) is a hormone produced during pregnancy.$\newline$ - hCG maintains the corpus luteum, which in turn produces estrogen and progesterone.$\newline$ - These hormones are crucial for maintaining the pregnancy.$\newline$ - Therefore, this option is correct.$\newline \bullet$Option 4: High level of hCG stimulates the thickening of endometrium.$\newline$ - While hCG supports the corpus luteum, it is progesterone that directly maintains the endometrium.$\newline$ - Therefore, this option is incorrect.$\newline$Thus, the correct option is Option 3: High level of hCG stimulates the synthesis of estrogen and progesterone.