Which combination is most suitable for making a clean green?

Correct answer: Clean yellow and clean blue

In traditional pigment colour mixing, green is produced by combining yellow and blue. Using clean yellow and clean blue is especially suitable because unwanted traces of red, black, or another pigment can dull the mixture and make it muddy. Red with yellow generally produces orange, red with blue produces violet, and yellow with black produces a darker or duller yellow rather than green. Therefore option C is correct.