The correct answer is: B. 500 metres
A W/L board is a wind/load board, which is a sign that indicates the maximum wind speed and load that a structure can withstand. It is important to have a W/L board at not less than 500 metres from the gate so that drivers can be aware of the potential hazards and take appropriate precautions.
Option A: 400 metres is too close to the gate. If a strong wind were to blow, the structure could collapse and injure or kill drivers.
Option C: 600 metres is too far from the gate. Drivers may not be able to see the W/L board in time to take appropriate precautions.
Option D: 800 metres is too far from the gate. Drivers may not be able to see the W/L board at all.
Therefore, the correct answer is B. 500 metres.