If a red-orange mixture appears too orange, what is the correct way to balance it toward red?

Correct answer: Add a little red

The governing concept is proportional colour mixing. Red-orange contains red and orange, and it appears too orange when the orange influence is relatively strong. Adding a small amount of red increases the red proportion and shifts the hue toward red. Yellow would strengthen the orange-yellow direction, while green or blue would introduce a different component and may dull or change the mixture. Thus option D is the controlled correction.