How can a yellow and violet mixture be balanced if it becomes too violet?

Correct answer: By increasing yellow

This question applies the principle of complementary mixing and proportional correction. Yellow and violet are complementary colours. If the mixture appears too violet, violet is present in excess or yellow is relatively insufficient; adding yellow gradually counteracts the violet and restores balance. Thus option D is correct. Blue or red would introduce a different hue, and black would darken the mixture without specifically correcting the complementary-colour imbalance.