Which of the following devices is prohibited on Frontier flights?

The correct choice is that radios are prohibited on Frontier flights due to regulations that restrict the use of devices which could interfere with the aircraft's navigation and communication systems. The Federal Aviation Administration (FAA) has stringent guidelines regarding electronic devices that might cause interference, and portable radios fit this category. They can emit signals that could disrupt the sensitive avionics onboard the aircraft.

In contrast, smartphones are generally permitted, although they must typically be switched to airplane mode during the flight. Hearing aids and pacemakers are essential medical devices that are allowed because they do not interfere with the aircraft systems and are necessary for the health and safety of passengers. Therefore, the limitation on radios specifically relates to safety procedures aimed at ensuring that all forms of electronic devices do not compromise the operation of the aircraft.