What may be added to balance a painting containing too many warm colours?

Correct answer: Some cool colours

The governing concept is colour balance through contrast and temperature. If a painting contains too much red, orange, or yellow, adding measured areas of cool colours such as blue, green, or violet can reduce the visual heat and create variety. More red or orange would intensify the same imbalance, while yellow is also generally warm and would not provide the needed temperature contrast. Therefore option A is correct, although the amount and placement should suit the composition.