Which of the following is NOT a type of health insurance plan?

The answer is D, IPO, because it is not recognized as a common category of health insurance plan. Health insurance plans typically fall into well-defined categories with specific structures and networks, and the commonly used types are represented by the other options.

Health Maintenance Organizations (HMOs) are plans that require members to choose a primary care physician (PCP) and obtain referrals to see specialists. This model focuses on preventative care and typically requires members to seek treatment from a network of providers.

Preferred Provider Organizations (PPOs) offer more flexibility in choosing healthcare providers, allowing members to see any doctor or specialist without needing a referral, although staying within the network usually results in lower costs.

Exclusive Provider Organizations (EPOs) also require members to use a network of providers but do not require referrals to see specialists. The key difference is that EPO plans do not cover any out-of-network care except in emergencies.

In contrast, IPO does not correspond to a recognized category of health insurance plan in the same way, hence making it the correct choice for what is NOT a type of health insurance plan.