What is a requirement for eligibility for stand-alone Medicare Prescription Drug Plans?

C is the correct choice because to be eligible for stand-alone Medicare Prescription Drug Plans (PDPs), an individual must be entitled to Medicare Part A and/or enrolled in Medicare Part B. This means that a person can qualify for these prescription drug plans if they have one or both of these components of Medicare coverage. The intention behind this requirement is to ensure that only those who are already part of the Medicare system can access additional benefits such as prescription drug coverage.

The other options do not fulfill the necessary criteria for eligibility. Enrollment in Part A only does not suffice, as it requires either Part A entitlement or Part B enrollment. A valid Social Security Number, while often required for various Medicare processes, is not a specific eligibility criterion for stand-alone PDPs. Lastly, there is no age restriction set at 75 years for eligibility in these plans; individuals under 65 who are entitled to Medicare due to disability may also enroll in a PDP, emphasizing that age is not a limiting factor in this context.