What should be added if a violet mixture appears too red?

Correct answer: A little blue

The governing concept is correcting a colour mixture by adding the component that is relatively lacking. Violet is formed from red and blue. If it appears too red, the mixture needs a small amount of blue to restore a better red-blue balance, so option A is correct. More red would intensify the error; yellow could dull or neutralise violet, and black would darken it rather than correct its hue.