Which option shows the clearest relative pair of a warm and a cool colour?

Correct answer: Yellow-orange and blue-violet

The governing concept is relative colour temperature. Yellow-orange is generally classified as warm, while blue-violet is generally classified as cool; their contrast makes the pair especially clear on a colour wheel. Red and orange are both warm, and blue with blue-green is mainly a cool pair. Yellow-green and yellow are also usually placed on the warm-to-neutral side rather than forming as strong a warm–cool pair. Therefore option A is the clearest answer.