What should be added to make violet appear more reddish?

Correct answer: Red

The governing concept is hue adjustment through controlled mixing. Violet already contains red and blue; adding a small amount of red increases the red component and shifts the colour toward red-violet, making it appear warmer and more reddish. Thus, option C is correct. Yellow or green would move the mixture away from a red-violet appearance, while black would mainly reduce lightness and may dull the colour rather than add red hue.