Which change is suitable for making violet made from red and blue cooler?

Correct answer: Increase blue

The governing concept is colour temperature within a hue family. Violet is made from red and blue; increasing the blue component shifts the mixture toward blue-violet, which is generally perceived as cooler. Therefore option C is correct. White mainly makes a tint, extra red makes the violet warmer, and yellow moves the mixture toward a duller neutral or brownish direction rather than reliably cooling it.