The correct answer is: B. Home Signal
A home signal is a railway signal that indicates whether a train can proceed into a section of track. In TALQ signalling system, the home signal is the first signal that a train encounters when approaching a station. The home signal will be cleared if the line is clear up to the home signal.
During thick & foggy weather, it is important to ensure that the line is clear up to the home signal before granting line clear. This is because it is difficult to see in thick fog, and there is a risk that a train could collide with another train or an object on the track.
The other options are incorrect because they are not the first signal that a train encounters when approaching a station.
A calling-on signal is a railway signal that indicates that a train can proceed into a section of track, but only after the train has received a clear to proceed from the signaller.
A shunt signal is a railway signal that is used to control the movement of trains within a yard or siding.
None of these is the first signal that a train encounters when approaching a station.