Which colour is mixed with violet to make blue-violet?

Correct answer: Blue

The relevant concept is tertiary-colour formation on the colour wheel. Blue-violet is formed from blue and violet, so option C is correct. Violet is already a secondary colour, and blue is the neighbouring primary colour used to create this tertiary hue. Red would move the mixture toward red-violet, while yellow or green would produce other relationships. The two words in the colour name identify the required components.