How should the brush be prepared while removing pooled colour from the lower edge of a large wash?

Correct answer: Clean and slightly damp

The relevant wash-control principle is controlled absorption. A clean brush that is only slightly damp can pick up the pooled pigment and water at the lower edge without adding a new wet patch or disturbing the whole wash. Thus, option D is correct. A loaded brush adds paint, an oversoaked brush returns water, and a dry stiff brush may scratch or lift the paper surface.