The background colour of the informatory sign board is A. red B. yellow C. green D. white

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.