Nearly all citizens of the U.

Detailed Answer in Step-by-Step Solution:

Medicare Part B eligibility generally requires enrollment in or eligibility for Part A (B), which covers hospital insurance and is tied to age (65+) or disability status, not just retirement (A) or insurability (C).

"Fully insured" under Social Security (D) relates to benefits but isn’t a direct Part B requirement.

The Virginia study guide states that Medicare Part B is available to those eligible for Part A, typically U.S. citizens or residents aged 65 or disabled, regardless of other factors. Reference: Virginia Life, Annuities, and Health Insurance study guide, section on "Medicare Eligibility."