The correct answer is: A. Repeating signal.
A repeating signal is a type of railway signal that is used to provide a clear indication of the safe route to a train driver when the main signal is not visible from the minimum sighting distance. Repeating signals are typically located at the entrance to a station or siding, and they are activated by the train driver when they approach the signal. The repeating signal will then display the same indication as the main signal, allowing the train driver to proceed safely.
A call-on signal is a type of railway signal that is used to request permission from a signaller to proceed. Call-on signals are typically located at the entrance to a section of track that is controlled by a signaller, and they are activated by the train driver when they wish to proceed into the section of track. The call-on signal will then display a green light, indicating that the train driver has permission to proceed.
A shunt signal is a type of railway signal that is used to direct a train onto a siding. Shunt signals are typically located at the entrance to a siding, and they are activated by the train driver when they wish to proceed onto the siding. The shunt signal will then display a green light, indicating that the train driver has permission to proceed.
None of these options are correct.