What should be increased to make blue-violet more violet?

Correct answer: Red

The governing concept is adjusting the proportion of neighbouring hues in a colour mixture. Blue-violet contains blue with a violet or red tendency; increasing red shifts the mixture toward a more balanced violet. Thus option B is correct. Yellow and green move the mixture away from violet, while white mainly raises its lightness and creates a tint rather than making it more violet. Only a small amount should be added at a time.