The correct answer is: D. white
The background color of an informatory sign board is white. This is because white is a neutral color that can be easily seen by drivers and pedestrians. It is also a color that is not associated with any particular danger or warning, so it is not likely to cause confusion.
The other options are incorrect because they are not neutral colors. Red is a color that is associated with danger and warning, so it would not be a good choice for an informatory sign board. Yellow is also a color that is associated with danger and warning, but it is less likely to cause confusion than red. Green is a color that is associated with safety and caution, so it would not be a good choice for an informatory sign board.