A mixture has the correct hue but its value is too low. What is the best way to lighten it without greatly changing the hue?

Correct answer: Add a little white

Value means the relative lightness or darkness of a colour. Since the hue is already correct and the mixture is too dark, a small amount of white is the standard way to raise its value and create a lighter tint. Option A is therefore correct. A complementary or third primary colour would alter the hue and may reduce saturation, while black would lower the value further.