What action must be taken if the person giving the signal to back a train disappears?

When a signal to back a train is given, communication and safety are paramount. If the person giving the signal disappears, the appropriate action is to stop movement if it is safe to do so. This ensures that there is no risk of moving the train without visual confirmation of the signal or the conditions that led to its original communication.

Stopping allows time to reassess the situation, ensuring that the train does not proceed without clear and continuous guidance. It is crucial in the railway environment to maintain strict safety protocols, and in the absence of a visual signal, the safest course is to halt operations until clarity can be reestablished.

The other options might compromise safety, such as continuing to move without acknowledged instructions or attempting to proceed cautiously without ensuring that the conditions are safe. Using the radio to verify could further delay the necessary response and does not address the immediate concern of stopping.