Among traditional primary colours which generally has the darkest natural value?

Correct answer: Blue

The governing concept is relative colour value. In the traditional red-yellow-blue set, blue generally appears darkest, yellow lightest and red intermediate when the colours are viewed at comparable strength. The exact result can vary with the pigment, but the question asks for the usual traditional comparison. White is not one of the traditional primary colours and has a light value. Thus option B, blue, is the best answer.