Under federal Medigap rules, what is the 'Medigap Open Enrollment Period' for a Medicare Part B enrollee?

Explanation

The federal Medigap Open Enrollment Period under 42 U.S.C. §1395ss is a ONE-TIME 6-month window that begins on the first day of the month in which the beneficiary is both age 65 or older AND enrolled in Medicare Part B. During this window, insurers must issue ANY Medigap plan they offer in the state on a guaranteed-issue basis, without medical underwriting and without surcharges for pre-existing conditions (subject to limited HIPAA-style lookback rules). After this window closes, future Medigap purchases are generally subject to medical underwriting unless a federal or state guaranteed-issue 'trigger' applies (e.g., loss of employer coverage). California layers a state-specific Birthday Rule under §10192.11 allowing annual same-or-lesser-benefit switches without underwriting. Options A, D, and B fabricate other windows.