Which of the following is a prohibited electronic device on board at all times?

Radio transmitters are considered prohibited electronic devices on board at all times due to their potential to interfere with the aircraft's communication and navigation systems. These devices can emit signals that may disrupt the operation of sensitive avionics systems, which are crucial for the safe operation of the aircraft. Consequently, airlines enforce strict regulations that prohibit their use in order to maintain safety for passengers and crew.

On the other hand, other devices like smartphones, laptops, and tablets are typically permitted onboard but are subject to specific regulations regarding their use during flight, such as being placed in airplane mode.