Final answer:
During the menses phase of the menstrual cycle, the old functional layer of the endometrium is sloughed off due to the decline in progesterone that follows the degradation of the corpus luteum.
Step-by-step explanation:
During the menstrual cycle's menses phase, which typically occurs during the early follicular phase of the ovarian cycle, several significant events take place. This phase is characterized by low levels of progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). This hormonal reduction triggers the shedding of the stratum functionalis, which is the compact layer of the endometrium that had been prepared for a potential pregnancy. As there is a decline in progesterone due to the degradation of the corpus luteum, this results in the shedding of the inner portion of the endometrium, which is observed as menstrual bleeding. Therefore, during the menses A) the old functional layer is sloughed off is the correct answer. Menses is not associated with a new uterine lining formation, high activity of the corpus luteum, development of secretory glands and blood vessels in the endometrium, or high progesterone levels; these occur in other phases of the menstrual cycle.