What is the main reason watercolour luminosity may decrease when many transparent layers are repeatedly applied over the same area?

Correct answer: Increasing pigment layers reduce the light reflected from the paper

Watercolour luminosity comes largely from light entering the transparent paint, reflecting from the white paper, and returning to the viewer. Repeated glazing deposits more pigment, so less paper brightness shows through; the colour can consequently look heavier, darker, or duller. Thus option A identifies the main cause. Water alone does not make the paper opaque, a brush cannot turn paint white, and layering does not automatically erase all paper texture.