When must the functioning of the illuminated marking device be confirmed?

The correct approach regarding the functioning of the illuminated marking device indicates that it must be confirmed at crew change locations. This is crucial because crew members need to ensure that all safety devices, including illuminated marking devices, are operational to maintain safety standards. Crew change locations are significant points where it's essential to verify equipment functionality, as this can affect the safety of operations as the train progresses on its journey.

Confirming the operation specifically at crew change locations allows for a standardized check where personnel transition, ensuring that the incoming crew is fully briefed and aware of the operational integrity of the train and its systems. This process directly contributes to avoiding any potential issues regarding visibility and signaling, which could arise from an inoperative device.

The other options, while they might sound plausible, do not align with the specific regulation governing when these checks are mandated. For instance, confirming the device's function before departure could be a good practice but is not explicitly required in the same way as at crew change locations, where there is a formal handover of operational responsibility.