What is the most important basis for considering a colour primary in traditional painting?

Correct answer: It cannot be made by mixing other colours

The governing concept is the traditional subtractive pigment model, in which primary colours serve as basic starting colours for mixing. Under this classroom convention, a primary colour is not produced by combining other colours within the same system; red, yellow, and blue are the familiar examples. Option A is correct. Brightness, natural origin, and identical appearance across media are not defining criteria.