Final answer:
The decline in estrogen and progesterone levels during the uterine cycle leads to menses, where the uterine lining is shed, marking the start of a menstrual period.
Step-by-step explanation:
When the levels of estrogens and progesterone decline in the uterine cycle, the event that occurs is called menses (C). This is the phase where the lining of the uterus, or endometrium, is shed through the vagina. This process is commonly known as a woman's menstrual period and signifies the beginning of a new menstrual cycle. The decline in hormone levels signals the body that there is no pregnancy, and as a result, the thickened lining of the uterus is no longer needed and is expelled.