What type of signal requires a train to stop short before requesting permission to pass?

The correct answer is associated with an automatic signal displaying 'Z' or 'D' because these signals indicate that the train must stop short and cannot proceed until clear authority has been granted. The 'Z' signal generally indicates that the track ahead is obstructed or unsafe, while the 'D' signal serves a similar purpose, emphasizing the need for caution and verification of track conditions before moving forward. This system is in place to ensure the safety of train operations and to maintain efficient traffic management on the rail network.

In contrast, a red signal universally means stop, and a yellow signal typically indicates caution or prepare to stop, but does not require a stop before requesting permission. A green signal indicates that it is safe to proceed, thus not necessitating any stopping or requesting permission. The requirement to stop short specifically pertains to the automatic signals designated as 'Z' or 'D', marking them as unique in conditions that necessitate additional safety protocols.