Final answer:
National health insurance refers to a tax-supported system where the government ensures citizens have access to basic health services, exemplified in the US by programs like Medicare and Medicaid, as well as the ACA.
Step-by-step explanation:
National health insurance is a mechanism by which healthcare services are provided to all citizens, typically supported through taxation. In the context of the United States, national health insurance manifests in programs such as Medicare and Medicaid. Medicare is a single-payer system funded by payroll taxes to provide health insurance to individuals 65 and older, and Medicaid is a government-funded insurance program providing healthcare to those with low incomes, funded by taxes. Additionally, the Patient Protection and Affordable Care Act (ACA), often referred to as Obamacare, is another health policy designed to increase the number of Americans with insurance by creating marketplaces and providing subsidies to make insurance more affordable.
The correct definition of national health insurance in this context is: a tax-supported mechanism in which the government guarantees a basic package of health services to all citizens. This includes government intervention in markets where failures occur, such as the inability to provide affordable insurance to low-income individuals or the elderly.