Which method is useful if paint is drying too quickly during colour filling?

Correct answer: Work in smaller sections and keep the mixture ready

The governing concept is controlled wet-edge painting. When paint dries quickly, working on a smaller section and preparing enough colour in advance allows adjacent strokes to meet while they are still workable. This produces a more even fill and makes blending easier. Heavy rubbing can damage the surface, a dry brush does not solve rapid drying, and stopping after every stroke creates visible joins. Therefore, option D is correct.