The background colour of the informatory sign board is

In highway engineering, various traffic signs are used to convey important information to drivers. The background color of informatory sign boards is typically white (Option D). White is commonly used for informational signs as it provides good visibility, especially during the day.

Option A: Red is often used for regulatory signs to indicate mandatory instructions or prohibitions.

Option B: Yellow is commonly used for warning signs to indicate potential hazards or cautionary information.

Option C: Green is typically used for guide signs that provide directional or distance information.

So, in the context of informatory sign boards, the correct background color is Option D: white.