The correct answer is: B. Automatic stop signal+120 Meters
The LSS (Lineside Shunting Signal) is a signal that is used to control the movement of shunting locomotives. It is located at the entrance to a shunting area and indicates whether or not the area is clear for shunting operations.
In automatic block area SL, the LSS shall not be taken off unless the line is clear up to the automatic stop signal plus 120 meters. This is to ensure that there is sufficient space for the shunting locomotive to clear the area before the next train arrives.
Option A is incorrect because it does not take into account the need for sufficient space for the shunting locomotive to clear the area.
Option C is incorrect because it is more than the required distance.
Option D is incorrect because it is not a valid option.