The correct answer is B. 600 meters.
Block overlap is the distance between the two signal indications at the entrance and exit of a block section. It is necessary to ensure that a train has cleared the block section before another train can enter it. The amount of block overlap required depends on the type of signalling system used. In a two-aspect signalling system, the block overlap is typically 600 meters. This is because a two-aspect signalling system uses two lights to indicate the track condition: green for clear and red for danger. If the block overlap were less than 600 meters, there would be a risk that a train could enter the block section while another train was still in it.
The other options are incorrect. Option A is 500 meters, which is less than the required block overlap for a two-aspect signalling system. Option C is 400 meters, which is also less than the required block overlap. Option D is 600 meters, which is the correct answer.