Step 1: The corpus luteum, formed after ovulation, secretes progesterone, making Assertion (A) true.
Step 2: Progesterone prepares and maintains the endometrium for pregnancy, making Reason (R) true.
Step 3: Since progesterone from the corpus luteum directly supports the endometrium, (R) correctly explains (A).
Thus, both (A) and (R) are true, and (R) is the correct explanation of (A).