If green appears too yellow what should be added to make it more blue-green?

Correct answer: Blue

The governing concept is shifting a mixed colour toward one of its component hues. A yellow-green mixture can be moved toward blue-green by adding a small amount of blue, so option D is correct. Red or orange would introduce warm hues and could muddy the mixture, while white would only make the existing colour lighter without giving it the desired blue-green direction. The blue should be added gradually for control.