A student mixes orange and yellow and labels the result red-orange. What is the main error?

Correct answer: The mixture was incorrectly named instead of yellow-orange

The governing naming rule is that a tertiary colour is identified by the two neighbouring hues used to form it. Orange mixed with yellow produces yellow-orange, because yellow is the primary hue named first in the direction of the mixture. Red-orange would require red and orange. Thus the student’s practical mixture may be valid, but its label is wrong, making option C correct.