Which change will make red-violet cooler?

Correct answer: Increase the amount of blue

The governing concept is directional colour mixing: adding a neighbouring hue shifts the mixture toward that hue. Red-violet contains both red and blue, but increasing the blue component moves it toward blue-violet, the cooler side of the spectrum. Therefore option C is correct. Yellow and orange generally introduce warmer influence, while more red strengthens the warm side instead of cooling the mixture.