A student calls the mixture of red and violet blue-violet. What is the error?

Correct answer: The student chose the wrong name instead of red-violet

A tertiary colour is conventionally named from the two adjacent colours used to form it, with the dominant or named component identified first. Mixing red with violet produces red-violet, not blue-violet, because blue is not one of the stated components. Blue-violet would require blue combined with violet. The student therefore selected the wrong hue name, making option C correct.