Which mixture is suitable for making red-orange?

Correct answer: Adding more red to orange

The governing concept is hue adjustment and the formation of an intermediate colour. Orange already lies between red and yellow; adding a larger proportion of red shifts its hue toward red and produces red-orange. Therefore option C is correct. Blue and green tend toward blue-green, while yellow and green produce yellow-green. Mixing yellow with violet, a complementary pair, usually reduces brightness and creates a dull neutral rather than a clear red-orange.