The correct answer is C. Shut.
A repeating signal is a signal that is used to repeat the indication of a preceding signal. A calling on signal is a signal that is used to indicate that a train is approaching a station. A shut signal is a signal that is used to indicate that a train is not allowed to proceed beyond the signal. A no signal is a signal that is not in use.
In a block line, a shut signal is used to indicate that a train is not allowed to proceed beyond the signal. This is done to prevent collisions between trains. The signal is typically a red light or a semaphore arm that is lowered across the track. When the signal is shut, the train must stop and wait for permission to proceed.
The other options are incorrect because they do not indicate that a train is not allowed to proceed beyond the signal. A repeating signal is used to repeat the indication of a preceding signal. A calling on signal is used to indicate that a train is approaching a station. A no signal is not in use.