The correct answer is: a) progesterone level falls.
Menstruation is a complex process regulated by hormones. After ovulation, the body produces progesterone, which helps prepare the uterine lining for a possible pregnancy. If the egg is not fertilized, progesterone levels will eventually fall, which triggers the shedding of the uterine lining, leading to menstruation. In contrast, if the egg is fertilized, progesterone continues to be produced to support the pregnancy, and menstruation does not occur. Therefore, a drop in progesterone is directly linked to the initiation of menstruation.