The correct answer is: A. 400 and 600 meters
Warning boards for neutral section shall be fixed at a distance of 400 and 600 meters from the beginning and end of the neutral section. This is to ensure that drivers are aware of the change in track conditions and can take appropriate action.
Option B is incorrect because it states that warning boards shall be fixed at a distance of 500 and 250 meters. This is not the correct distance.
Option C is incorrect because it states that warning boards shall be fixed at a distance of 300 and 600 meters. This is not the correct distance.
Option D is incorrect because it states that none of the above options are correct. This is not the correct answer.