Which tertiary colour is considered complementary to yellow-orange?

Correct answer: Blue-violet

The governing concept is complementary placement on the colour wheel. In a standard twelve-part wheel, yellow-orange is opposite blue-violet, so these two colours produce the strongest complementary contrast. Red-violet is opposite yellow-green, and blue-green is opposite red-orange. Yellow-green is adjacent to yellow-orange rather than opposite it, so it is not the correct complementary partner.