What is useful for creating an opaque solid colour effect with a brush?

Correct answer: Thick colour with sufficient even coverage

The governing concept is opacity, meaning that the applied paint hides most or all of the surface beneath it. A sufficiently thick, evenly spread layer gives a solid colour effect, so option B is correct. Excess water makes paint more transparent, a wet surface alone contains no pigment for coverage, and strokes without paint cannot create any coloured layer. Even application also prevents unwanted gaps and streaks.