What indicates you should proceed prepared to stop at the next signal?

The correct choice is the approach signal. An approach signal indicates that the train must be prepared to stop at the next signal. This type of signal is typically used to indicate that the next signal is not clear, which could mean that it is a stop signal or requires the train to proceed at a restricted speed.

When a train sees an approach signal, it should reduce speed and be ready to bring the train to a halt if necessary. This enhances safety by ensuring that crews are aware that there may be upcoming changes in signal indications or track conditions that require their full attention and immediate action. Thus, the approach signal plays a crucial role in train operation protocols by keeping operators informed and prepared for the signals that follow.