Which precaution is necessary to keep watercolour layers luminous?

Correct answer: Apply each layer with minimal strokes

The governing concept is preserving the transparency and reflective brightness of watercolour. Applying each layer with a few deliberate strokes avoids disturbing the paper surface and prevents lower colours from becoming muddy through excessive mixing. Allowing a layer to settle or dry before adding the next can also help. Option A is correct; repeated scrubbing damages luminosity, black dulls colour, and oil is incompatible with a clean watercolour surface.