The correct answer is: D. Before 10 minutes from expected arrival.
The departure signal shall be taken off for run through train passing via loop line before 10 minutes from expected arrival. This is to ensure that the train has enough time to clear the station and reach the loop line before the next train arrives.
Option A is incorrect because the departure signal should not be taken off before the train enters the station. This is to prevent the train from accidentally leaving the station before it is ready to depart.
Option B is incorrect because the departure signal should not be taken off after the train has stopped at the station. This is to prevent the train from accidentally leaving the station before it has been properly unloaded and loaded.
Option C is incorrect because the departure signal should not be taken off after the train has entered the loop line. This is to prevent the train from accidentally leaving the loop line before it is ready to depart.