The correct answer is: Red hand signal.
A cabin master shall not show any other signal from his cabin during shunting except a red hand signal. This is because a red hand signal is the only signal that can be used to stop a train. If a cabin master were to show any other signal, it could lead to a collision.
A yellow signal is a caution signal. It means that the train must slow down and be prepared to stop. A green signal is a clear signal. It means that the train can proceed at normal speed.
A red hand signal is a stop signal. It means that the train must stop immediately. If a cabin master were to show a yellow or green signal during shunting, it could lead to a collision. This is because the train would not be expecting to stop and would not be able to stop in time.
Therefore, the correct answer is: Red hand signal.