c. He is not eligible to enroll in a Medicare Advantage plan until he re-enrolls in Medicare Part B.
To enroll in a Medicare Advantage (MA) plan, individuals must be enrolled in both Medicare Part A and Part B. Since Mr. Castillo has stopped paying his Part B premium, he is no longer enrolled in Part B. In order to be eligible for a Medicare Advantage plan, he would need to re-enroll in Medicare Part B first. Once he has both Part A and Part B coverage, he can then consider enrolling in a Medicare Advantage plan if he wishes to do so.