What is the best way to create a bright white detail over a dark dry acrylic base?

Correct answer: Apply a controlled layer of opaque white with a clean small brush

The governing concept is opacity and controlled layering in acrylic painting. A dry dark base should be covered with an opaque white paint layer, applied carefully with a clean small brush. The brush preserves the shape and sharpness of the detail, while the opaque pigment hides the dark colour. Excess water would make the white transparent and weak; rubbing on a wet base can disturb the underlayer, and mixing white into the dark area would remove the intended detail. Therefore, option C is correct.