Which correction is suitable if a poster colour has the correct hue but its value is too dark?

Correct answer: Add white in a controlled amount

The governing concept is value: value describes how light or dark a colour appears, while hue identifies its basic colour family. Since the hue is already correct and only the value is too dark, adding a small, controlled amount of white raises the value and lightens the mixture. Adding black would darken it further, while a complementary or three-primary mixture could change the hue and make the colour dull. Therefore, option C is the precise correction.