What correction can be made in poster paint if the desired colour becomes too dark?

Correct answer: Add a little white paint

The governing concept is value, which describes how light or dark a colour appears. If poster paint becomes too dark, adding a small quantity of white raises its value and produces a lighter tint. Thus, option C is correct. Black would lower the value further, while adding a complementary colour may dull or change the hue rather than simply lighten it. The colour need not be discarded.