It totally depends on speed at which train is moving.
If the speed of train is below 100 miles per hour the minimum distance maintained should be 4 feet.
If the speed of train is above 100 miles per hour and less than 125 miles per hour the minimum distance maintained should be 6.6 feet.(Approx)
As , when a train is moving with velocity more than this i.e above 125 miles per hour the minimum distance should be 15 feet.(Approximately)
Option (C) 15