Which colour change will increase the warmth of red the most?

Correct answer: Adding yellow

On a conventional colour wheel, adding yellow shifts red toward orange, a warmer neighbouring hue. The resulting red-orange mixture has a stronger warm bias, so option A is correct. Adding blue usually pushes red toward a cooler violet direction; violet also does not strengthen the yellow-orange warmth, and grey reduces chromatic intensity rather than increasing warmth. The exact result depends on pigments, but the directional principle remains clear.