Which signal requires a train to immediately begin speed reduction if exceeding Medium Speed?

The correct answer is associated with the need for a train to begin a speed reduction when it encounters a situation where it is exceeding Medium Speed. The "Approach Sign" serves as a warning to the engineer that they must be prepared to reduce speed in anticipation of another signal that may require them to stop or lower their speed significantly. This type of signal indicates that the next signal is not clear, and the engineer must begin to slow down.

In railway operations, maintaining proper speeds is critical for safety and efficiency, particularly in areas where trains may encounter other traffic or need to negotiate switches. The Approach Signal ensures that engineers have adequate time to decelerate, thereby preventing potential collisions or safety incidents.

The other options represent different types of signals with distinct meanings and requirements. For instance, "Terminal Proceed" typically indicates that the train can proceed at a certain speed without needing to reduce it immediately, while "Terminals Restricting" signals would provide guidance for restricted speeds but not necessarily prompt immediate speed reduction in the same way. "Absolute Block Signal" indicates a complete stop or that the track ahead is blocked and requires strict adherence, which could involve a more absolute cessation of speed rather than a gradual reduction as indicated by the Approach Sign.