“Part C refers to the Medicare Advantage program, through which beneficiaries can enroll in a private health plan, such as a health maintenance organization (HMO) or preferred provider organization (PPO), and receive all Medicare-covered Part A and Part B benefits and typically also Part D benefits. Enrollment in Medicare Advantage plans has grown over time, with more than 20 million beneficiaries enrolled in Medicare Advantage in 2018, or 34 percent of all Medicare beneficiaries (Figure 3).”

Source: Issue Brief: An Overview of Medicare. Kaiser Family Foundation. February 2019. http://files.kff.org/attachment/issue-brief-an-overview-of-medicare