Menstruation flow occurs due to a lack of

The menstrual cycle is a complex process and is controlled by so many various glands. Menstruation flow occurs due to a lack of Progesterone. Some various factors can also affect the menstrual flow of a person and can make their period unusually light. The healthy body weight, exercise, and free from stress can result in the light periods. The periods that are lighter than usual are not normally a result of concern. The function of progesterone is to maintain the pregnancy and endometrium, its withdrawal results in abortion and menstrual flow, respectively. 

Therefore, menstruation flow occurs due to a lack of Progesterone.

Hence the correct answer is option A.