Step 1: Analyzing the assertion:
- The corpus luteum is a temporary endocrine structure in the ovaries that forms after ovulation.
- It secretes the hormone progesterone, which is essential for the regulation of the menstrual cycle and for the maintenance of the endometrium during pregnancy.
Step 2: Analyzing the reason:
- Progesterone is indeed necessary for the maintenance of the endometrium. It prepares the uterine lining for possible implantation of a fertilized egg and helps in maintaining the lining during early pregnancy.
Step 3: Conclusion:
- Both Assertion (A) and Reason (R) are true, and the reason provides the correct explanation for the assertion.
Thus, the correct answer is option (A).