Which process helps in making a clean green colour?

Correct answer: Mix blue and yellow on a clean palette

The governing mixing rule is that green is a secondary colour made by combining the primary pigments blue and yellow. Using a clean palette keeps red, brown, or leftover pigment from contaminating the mixture and dulling its appearance. Red and blue produce violet, red and yellow produce orange, and adding black only darkens existing green. Hence, option D is the complete and most suitable process.