Which colour pair can give strong contrast?

Correct answer: Red and green

The governing concept is complementary colour contrast. On a traditional colour wheel, red and green lie opposite each other, so their differences in hue can produce a strong, attention-grabbing contrast when placed together. Therefore option A is correct. Red–orange, blue–green, and yellow–orange are neighbouring or closely related hues, so they generally create more harmonious or lower contrast relationships. Actual strength also depends on value and saturation.