Which brush is useful for blending two wet colours?

Correct answer: Clean damp brush

The relevant brush technique is to soften the meeting edge while both colours remain workable. A clean, slightly damp brush can move lightly across that edge and merge the pigments with control, so option D is correct. A dry stiff brush may scratch or disturb the surface, while a colourless pencil and a metal tool are not suitable for gently blending wet paint. Excess water should also be avoided.