Which option shows the clearest warm and cool colour contrast?

Correct answer: Orange and blue

The governing concept is warm–cool colour contrast: warm colours such as red, orange and yellow generally suggest heat or energy, while cool colours such as blue and violet suggest calmness or distance. Orange and blue belong to opposite temperature groups, so their difference is clearer than the pairs of neighbouring warm or cool colours. Therefore option B is correct; the other pairs are both warm or both cool.