The correct answer is: A. Permissive.
A permissive signal is a signal that allows a train to proceed after it has been given the proper authority. A home signal is a signal that indicates whether or not a train is allowed to proceed into a station. A subsidiary signal is a signal that is used to supplement the main signal.
A permissive signal is a type of railway signal that indicates that a train is allowed to proceed after it has been given the proper authority. Permissive signals are typically used in conjunction with other types of signals, such as home signals and subsidiary signals.
A home signal is a type of railway signal that indicates whether or not a train is allowed to proceed into a station. Home signals are typically located at the entrance to a station and are used to control the flow of traffic into and out of the station.
A subsidiary signal is a type of railway signal that is used to supplement the main signal. Subsidiary signals are typically used to indicate the presence of a hazard or to provide additional information to the train crew.
In the context of the question, the calling on signal is a permissive signal. This means that the train is allowed to proceed after it has been given the proper authority. The other options are incorrect because they do not accurately describe the type of signal that is being discussed.