Which colour relationship is required to make red-orange?

Correct answer: A mixture of red and neighbouring orange

The governing colour-wheel relationship is adjacency. Red-orange lies between red and orange, so it is made by combining those neighbouring hues in a controlled proportion. Option A is correct because it names the two colours that surround the target tertiary hue. Red with complementary green, or orange with complementary blue, tends to reduce saturation and create a duller mixture; a distant blue does not form red-orange.