What should an artist do to soften the boundary between two wet colours?

Correct answer: Gently blend the boundary with a clean damp brush

The governing concept is wet blending: colour transitions become softer when adjacent wet pigments are gently joined before they dry. A clean, slightly damp brush loosens and merges the boundary without adding unwanted colour, so option D is correct. A dry cloth can disturb the surface, a black line makes the edge harder, and allowing the colours to dry preserves a sharp boundary.