What key element is required for HMOs in relation to subscriber fees?

Health Maintenance Organizations (HMOs) operate on a model where subscribers pay a set fee at regular intervals, which is a core characteristic of their structure. This fixed periodic fee allows HMOs to manage healthcare costs effectively and provide services to their members. By charging this predictable subscription-like fee, HMOs can maintain a budget for the wide range of medical services they offer, which can include preventive care, hospitalization, and specialist services. This payment structure encourages subscribers to utilize healthcare services more often, thereby promoting a focus on preventive care and overall health management.

In contrast, the other options do not align with how HMOs function. Offering services for free would not be feasible for sustaining operations. A one-time enrollment fee does not facilitate ongoing care and management of services. Allowing payment plans over several years does not reflect the HMO model, which requires regular, consistent funding to provide continuous access to healthcare services.