What should be added to turn yellow-orange into a more balanced orange?

Correct answer: A little red

The governing concept is shifting a hue by changing the ratio of its component colours. Yellow-orange contains more yellow than a balanced orange; adding a small amount of red moves the mixture toward an equal red-yellow relationship. Thus option A is correct. White would only lighten it, while blue and green would introduce contrasting or neighbouring influences that do not produce a balanced orange. Small additions allow careful control.