A PPO plan is best defined as:

A PPO, or Preferred Provider Organization, is best defined as a type of managed care. This definition captures the essential characteristic of PPOs, which are designed to provide health care services through a network of preferred providers. Members of a PPO plan are given the flexibility to choose healthcare providers and have the option to see specialists without needing a referral. The "managed care" aspect is significant because it indicates that there is a structured network in place aimed at controlling costs while maintaining quality care.

The nature of PPOs as managed care plans also means they often negotiate lower rates with providers in their network, making it more economical for members to receive care through these preferred providers. Unlike other insurance models that may involve different levels of flexibility or restrictions on the choice of providers, PPOs balance patient choice with cost control, which is a hallmark of managed care systems.

The other options refer to different categories unrelated to the concept of managed care. Whole life insurance relates to a form of permanent insurance coverage, health savings accounts are savings tools that allow individuals to save for medical expenses, and disability insurance provides income in the event of an inability to work due to a disability. Each of these has distinct purposes and functions that do not align with the definition of a PPO plan.