According to the textbook:

• Assertion (A) is True: The corpus luteum secretes progesterone which is essential for the maintenance of the endometrium. Such an endometrium is necessary for the implantation of the fertilised ovum .
• Reason (R) is True: In the absence of fertilisation, the corpus luteum degenerates. This causes the disintegration of the endometrium leading to menstruation .
• Logic: While both statements are factually true, the Reason describes the mechanism of menstruation (loss of endometrium due to lack of fertilization), which does not explain why the endometrium is necessary for implantation (which is due to its nutritive and structural role).