If a mixed colour has the correct hue but its value is too low, which correction is most logical?

Correct answer: Add a suitable amount of white or lightening medium

The governing concept is separating hue from value. Hue identifies the colour family, while value describes how light or dark it is. Since the hue is already correct and the value is too low, the correction should raise lightness without unnecessarily changing the colour family. A measured amount of white or a suitable lightening medium does this. Black lowers value, and changing the hue or adding a complement addresses a different problem. Thus option C is correct.