The correct answer is 400 meters.
The distance between outer and home signals on a single line is 400 meters when there is no advanced starter or SLB in the opposite direction. This is the minimum distance required to allow a train to stop safely if the signal ahead is at red.
Option A, 900 meters, is the distance between outer and home signals on a single line when there is an advanced starter or SLB in the opposite direction. This is the minimum distance required to allow a train to stop safely if the signal ahead is at red and there is another train approaching from the opposite direction.
Option B, 800 meters, is the distance between outer and home signals on a double line. This is the minimum distance required to allow a train to pass another train on the opposite track.
Option C, 700 meters, is not a valid answer.