Final answer:
The Open Enrollment Period for Medigap Plans does not require underwriting and applies to Plans A and B, beginning when you're 65 or older and enrolled in Medicare Part B. This period lasts six months.
Step-by-step explanation:
The enrollment period that does not require underwriting for Medigap Plans is the Open Enrollment Period. This period starts on the first day of the month that you are both 65 or older and enrolled in Medicare Part B. During this time, you have a guaranteed right to buy any Medigap policy sold in your state without underwriting. The Medigap Open Enrollment Period lasts for six months. However, not all Medigap plans may be available in every state.
As for the specific answer to the multiple-choice question, the correct option is: A) Open Enrollment Period for Plans A and B. It's important to note that individual states can also have additional rules about switching plans or enrolling in Medigap outside of this period.