Why will a small red circle attract more attention on a calm blue background?

Correct answer: Because of warm–cool contrast and the advancing effect of red

The governing concept is contrast combined with visual emphasis. Red is generally warm and blue is generally cool, so their temperature difference creates a strong warm–cool contrast. Red also tends to appear visually active or advancing, allowing even a small circle to become a focal point against the quieter blue field. The colours do not have the same temperature, blue does not hide red, and red is not restricted to backgrounds; therefore option A is correct.