What does a "stop signal" indicate for train movement?

Answer: C — That a train must halt and cannot proceed until authorization is received

A "stop signal" is a critical command in train operations. It indicates that the train must come to a complete halt and cannot proceed until the crew receives proper authorization to continue. This signal is crucial for maintaining safety on the tracks, as it ensures that a train does not move into an area where there may be obstructions, other trains, or maintenance work taking place. The requirement for the train to stop is a fundamental aspect of railroad signaling protocols, emphasizing the importance of communication and adherence to safety procedures to prevent accidents. In contrast, the other options suggest various levels of movement that do not align with the strict directive of a stop signal, as they imply proceeding in some capacity rather than halting completely.