The correct answer is: C. Calling-on.
A calling-on signal is a railway signal that is used to authorize a train to proceed into a section of track. It is typically a green light or a green flag, and it is displayed by the signalman at the entrance to the section of track. The calling-on signal is the final authority for a train to proceed, and it must be obeyed by the driver.
A shunt signal is a railway signal that is used to authorize a train to move around within a yard or station. It is typically a yellow light or a yellow flag, and it is displayed by the signalman at the entrance to the yard or station. The shunt signal is not an authority for a train to proceed onto the main line, and it must not be obeyed by the driver if it is displayed in conjunction with a red signal.
A private number is a telephone number that is used by railway staff to communicate with each other. It is not a signal, and it should not be used to authorize a train to proceed.
None of these options is correct. The correct answer is: C. Calling-on.