Final answer:
Mary's Medicare Supplement Open Enrollment period is in September 2021.
Step-by-step explanation:
The Medicare Supplement Open Enrollment period is a specific time when individuals who are eligible for Medicare can enroll in a Medicare Supplement plan without being subject to medical underwriting. The Open Enrollment period starts when you turn 65 and are enrolled in Medicare Part B.
In the case of Mary, who turns 65 on September 23, 2021, and has a Medicare Part B effective date of September 1, 2021, her Medicare Supplement Open Enrollment period would be in the month of September 2021. During this period, Mary can choose any Medicare Supplement plan available in her area, and insurance companies are required to accept her application without any health questions or pre-existing condition exclusions.