Final answer:
The portability principle of Medicare ensures beneficiaries can use their coverage nationally with participating providers, indicative of mobile healthcare services.
Step-by-step explanation:
The portability principle of Medicare refers to the ability of beneficiaries to use their Medicare benefits across different states within the United States. This means that Medicare provides coverage for its beneficiaries regardless of where they are located within the country, as long as the healthcare providers they visit participate in the Medicare program. The correct answer to the question is B) Mobile healthcare services, which suggests that Medicare recipients have the flexibility to receive care from different providers while traveling or moving.