If adding white makes a colour look chalky and dull, which correction is useful?

Correct answer: Add a small amount of the base colour back

The governing concept is tint correction. White lightens a colour, but too much white can reduce saturation and make the mixture appear chalky. Adding a small amount of the original base colour restores some hue strength and liveliness, so option C is correct. More white would worsen the problem, much black would darken and muddy it, and an equal complementary quantity could neutralise the hue too strongly.