The correct answer is: B. 500 meters
Whistle boards are devices that are used to warn motorists of the approach of a train at a level crossing. They are typically made of metal or plastic and are mounted on poles or other structures near the crossing. When a train approaches, the whistle board will emit a loud whistle sound to alert motorists to the danger.
The distance at which whistle boards must be placed in rear of all level crossing gates on both side is 500 meters. This distance is specified in the Indian Railway Safety Rules, 1989. The rules state that “whistle boards shall be provided at an adequate distance in rear of all level crossing gates on both sides.” The purpose of the whistle board is to warn motorists of the approach of a train and to give them time to stop before reaching the crossing.
The other options are incorrect because they are not the distances specified in the Indian Railway Safety Rules, 1989.