Which signal aspect requires preparation to stop at the next signal?

The aspect that indicates preparation to stop at the next signal is "Proceed with caution." This signal aspect alerts the train operator that there may be a condition ahead requiring the train to slow down and prepare to stop, without immediately directing the operator to do so. It serves as a warning that the next signal could present a more restrictive indication, such as requiring a full stop. Therefore, the operator must remain vigilant and ready to respond appropriately.

In contrast, the other signal aspects convey different meanings surrounding the operation of the train. A clear signal generally indicates that the track ahead is clear, allowing the train to continue without stopping. "Stop and call" instructs the operator to bring the train to a full stop and communicate with dispatch or control. "Normal proceed" means that the train can continue at normal speed without any restrictions. Thus, "Proceed with caution" stands out as the signal that mandates readiness to halt at the subsequent signal.