Which experiment would demonstrate that yellow is a primary colour in the traditional pigment system?

Correct answer: Failing to make yellow by mixing red and blue

The governing concept is the definition of a primary pigment colour: within a specified system, it cannot be obtained by mixing the other basic pigments. In the traditional red-yellow-blue model, repeated attempts to create yellow from red and blue fail, supporting its independent status. Therefore option A is best. Making green, a tint, or a covered mark demonstrates other properties but does not establish yellow as primary.