What should be added to make red-violet more reddish?

Correct answer: Red

Red-violet is a mixed colour containing red and violet, with violet itself containing blue and red. To make the mixture visibly more reddish, increase the amount of red; therefore option A is correct. Adding blue would strengthen the blue-violet direction, while green or yellow would alter the mixture toward other hues rather than directly increasing its redness. The governing rule is to add more of the desired component.