If a dark line forms at the edges of a dried watercolour layer what correction is needed?

Correct answer: Gently lift excess pigment at the edge with a clean damp brush to soften the line

The governing cause is pigment accumulation at a drying boundary, often called a hard or dark edge. A clean, damp brush can gently lift some excess pigment from that boundary; blotting the brush between touches prevents spreading too much colour. Thus option D is the suitable correction. Adding black increases the defect, scratching damages paper fibres, and uncontrolled rewetting may create blooms or enlarge the mark.