What should be done to make blue-violet appear bluer?

Correct answer: Increase blue

The governing concept is colour proportion in a mixture. Blue-violet contains blue and violet, so increasing the amount of blue makes blue the stronger visual influence and shifts the mixture toward a bluer appearance. Therefore, option B is correct. Adding red or yellow would move the colour away from pure blue, while adding green would create a duller, different mixture rather than a clearly bluer one.