The correct answer is the "thickened uterine lining". The thickened uterine lining or the endometrium is shed during menstrual period. In the event wherein fertilization does not occur, the "bed" of endometrium for the zygote will have no use and because of the lack of stimulation from the hormones (progesterone levels will eventually decrease), there will be shedding of the endometrium leading to the menstrual period.
