What should be added to shift red-violet toward red?

Correct answer: Red

The governing concept is directional colour mixing. To shift an existing red-violet mixture toward red, add more of the hue named as the desired direction. Adding red increases the red component and makes the resulting colour appear more reddish. Blue would strengthen the violet or cool influence, green would muddy the mixture, and yellow would move it toward a warmer yellowish effect rather than directly toward red. Thus, option B is correct.