What is the best way to maintain maximum saturation in a secondary colour?

Correct answer: Use clean primary colours with compatible biases

The governing concept is pigment bias and saturation. A secondary colour is most vivid when its two primary pigments are clean and lean toward the desired result; for example, warm red plus warm yellow can produce a cleaner orange. Adding a complement, grey, or the third primary introduces neutralising colour and reduces chroma. Therefore option A is correct, while B, C, and D make the mixture duller.