Which tertiary colour uses orange as the secondary colour and red as the primary colour?

Correct answer: Red-orange

The governing concept is tertiary colour construction: a primary colour is mixed with the adjacent secondary colour. Red is the primary and orange is the secondary named in the question, so their mixture is red-orange, making option A correct. Yellow-orange uses yellow with orange; red-violet uses red with violet; and blue-green uses blue with green. Identifying both components removes the distractors.