When is a sufficiently thick layer more useful than repeated thin layers in opaque colour filling?

Correct answer: When the lower surface must be fully covered

The governing concept is opacity: a suitably thick paint layer contains enough pigment to hide the surface beneath it. Option A is correct when the lower drawing, stain, or previous colour must be fully covered in an even application. Transparency requires a thinner mixture, flowing paint is controlled by fluidity rather than thickness alone, and soaking the paper is unrelated. The layer should still remain workable, not excessively stiff.