What is necessary for applying white dots over a dark poster colour?

Correct answer: The dark colour layer should be dry and the white paint sufficiently thick

The governing concept is opaque layering. The dark underlayer must be dry so that the new white marks do not mix with it or spread across the surface. White poster colour should be sufficiently thick to provide opacity and contrast against the dark background. A wet base, watery white, or mixed colours would weaken the dots and make them blurred or dull. Hence option A gives the conditions needed for crisp, visible white dots.