Menstruation is triggered by a drop in the levels of:
a. fsh.
b. lh
c. relaxin.
d. progesterone.
e. inhibin.

The answer is: d. Progesterone.

Explanation:

The menstrual cycle consists of a series of regular changes that naturally occur in the female reproductive system.

When the follicular phase begins, estrogen and progesterone levels are low. As a consequence, decomposition and shedding of the upper layers of the uterine lining, endometrium, and menstruation occurs.

The answer is: d. Progesterone.