What is the best basis for distinguishing yellow-orange from red-orange?

Correct answer: Yellow dominates the first while red dominates the second

The governing idea is hue bias within tertiary colours. Yellow-orange is an intermediate colour that leans from orange toward yellow, whereas red-orange leans from orange toward red. Thus, the first colour has a stronger yellow influence and the second has a stronger red influence. Neither colour is simply a primary or secondary colour, and both are located on the same colour wheel. Therefore, option A is correct.