How must a train acknowledge a stop signal that is not a fixed signal?

The correct response is rooted in the established protocols for train operations when a train encounters a stop signal that is not a fixed signal. Acknowledging a stop signal through the procedures outlined in rule 14a or 14b is crucial for ensuring safety and compliance with operational rules.

These rules detail specific actions that the crew must take to acknowledge signals effectively. This rigor ensures that all personnel on the train are aware of the signal's status and reinforces communication within the crew without relying on potentially confusing or disruptive methods.

Utilizing the established signaling rules allows for a standardized approach to such situations, minimizing the risk of misunderstanding and enhancing safe train operations. It also helps maintain efficiency within the rail system, as it promotes consistent behaviors in response to signals.

In contrast, options like sounding a horn continuously or reversing the train introduce ambiguity and potential hazards. Such actions could distract or confuse other train crews or lead to unsafe operational practices. Therefore, adhering to the specific acknowledgment procedures in rule 14 is paramount in this context.