How are red and green generally related on a colour wheel?

Correct answer: They are complementary colours

The governing concept is complementary colour relationships. On the traditional RYB colour wheel, red is a primary colour and green is made by mixing yellow and blue; the two are positioned opposite each other. Opposite colours create strong contrast and are called complementary colours, so option B is correct. They are not both primary, although green is secondary, and they are not merely lighter or darker versions of one hue.