Which of the following is designed to pay qualified medical expenses for individuals who are covered by high-deductible health plans?

A flexible spending account (FSA) allows employees to set aside pre-tax dollars for medical expenses, but it is not specifically tied to high-deductible health plans. FSAs are often employer-sponsored and have a "use-it-or-lose-it" policy, requiring funds to be used within a certain time frame, which differs from the flexibility of HSAs.

Medicare supplement insurance, also known as Medigap, is designed to cover costs not included in Medicare, such as copayments and deductibles. It does not pertain to high-deductible health plans and is specifically for individuals already enrolled in Medicare, making it irrelevant to the question at hand.

Long-term care insurance provides coverage for services that assist with activities of daily living, typically over an extended period. This type of insurance does not specifically address high-deductible health plans or cover qualified medical expenses in the same manner as HSAs, focusing instead on long-term care needs.