Which mixture can create a light cool grey?

Correct answer: Add a little blue to a white-dominant grey

The relevant concepts are value and colour temperature. Begin with a grey containing mostly white so that its value remains light. Then add only a small amount of blue; blue shifts the neutral mixture toward a cool temperature without making it strongly coloured. Option A therefore satisfies both conditions. Yellow in black is not a controlled light grey, while white-red and black-orange produce different hues rather than a cool neutral.