Why should the palette knife or brush be cleaned before making a new poster-colour mixture?

Correct answer: To prevent old colour from entering the new mixture

The governing concept is maintaining colour purity during mixing. Residual paint on a palette knife or brush can enter the next mixture and change its hue, brightness, or value, making the result muddy or different from the intended colour. Therefore option D is correct. Cleaning does not dry paint faster, thicken water, or enlarge the palette; it prevents unwanted contamination and gives predictable mixtures.