The hormone responsible for regulating the ovarian and menstrual cycle is ________. (a) human chorionic gonadotropin. (b) prolactin. (c) testosterone. (d) estrogen.

The correct answer is (d) estrogen. Estrogen is produced by the ovaries and plays a crucial role in regulating the menstrual cycle, including the growth of the uterine lining and the release of an egg during ovulation. It also helps to maintain pregnancy and plays a role in the development of secondary sexual characteristics in females. Human chorionic gonadotropin (a) is a hormone produced during pregnancy, prolactin (b) is responsible for milk production, and testosterone (c) is primarily produced in males and plays a role in the development of male sexual characteristics.