If a blue-green mixture has the correct hue but is too dark what is the best way to lighten it?

Correct answer: Add white gradually

The governing concept is separating hue from value. Hue identifies the colour family, while value identifies its lightness or darkness. Since the blue-green hue is already correct, gradually adding white raises the value without intentionally introducing a complementary colour. Option A is therefore best. Red-orange could neutralise or shift the hue, black would darken it further, and excessive yellow would change the colour balance.