Which of the following is true of the proliferative phase of the menstrual cycle? a. The endometrium thickens in response to increased estrogen. b. The cervical mucus thickens and starts forming a plug to seal off the uterus. c. Glands within the uterus enlarge and begin secreting glycogen. d. The corpus luteum begins to degenerate as hormone levels decline.

Option A, The endometrium thickens in response to increased estrogen

Explanation:

During the proliferative phase of the menstrual cycle, the following events occurs

a) Development of follicles inside the ovaries

b) Follice mature for ovulation.

c) Levels of FSH increase

d) The uterus prepares itself for implantation.

The mature follice releases estrogen that causes the growth of glands along with thickening of endometrium thereby thickening the uterine lining

Hence, option A is correct