What should be added if an orange mixture has become too yellow?

Correct answer: A little red

The governing concept is correcting a mixture by restoring its colour balance. Orange is produced from red and yellow. If the mixture contains too much yellow visually, adding a small amount of red shifts the balance back toward orange. Therefore, option D is correct. Blue may neutralise or muddy the mixture, green introduces another hue, and white only makes the colour lighter without correcting the yellow-red balance.