What change is expected when yellow is gradually blended into a cool blue?

Correct answer: The colour will shift toward green

The governing concept is gradual secondary-colour mixing. Yellow and blue are primary colours that combine in paint to produce green; as increasing amounts of yellow enter the blue, the mixture shifts from blue toward blue-green and then greener tones. Thus option C is correct. Purple requires red and blue, red is not produced by this pair, and black is not the expected result of a clean, controlled yellow-blue blend.