The correct answer is A. Fixed stop signal.
A fixed stop signal is a railway signal that indicates to a train that it must stop before proceeding. It is usually a red light, but may also be a semaphore arm or a combination of both. Fixed stop signals are located at the entrance to a station or block section, and they are the last signal that a train must obey before entering that area.
A hand signal is a signal that is given by a railway worker using their hands. It is used to control the movement of trains, and it can be used to indicate to a train to stop, proceed, or slow down. Hand signals are usually given by a flagman, but they can also be given by a train dispatcher or a stationmaster.
A call-on signal is a signal that is given to a train by a railway worker to indicate that it is clear to proceed. It is usually a green light, but it may also be a semaphore arm or a combination of both. Call-on signals are located at the entrance to a station or block section, and they are the first signal that a train must obey after leaving that area.
None of these is the correct answer because it does not describe a fixed stop signal.