What should be done if a poster-colour mixture is too thick?

Correct answer: Add a little water

The governing concept is adjusting paint consistency for application. If poster colour is too thick, add a small amount of water and mix thoroughly; repeat gradually until the paint spreads smoothly and evenly. Black changes darkness, white changes the colour’s value, and a complementary colour changes the hue, but none properly corrects thickness. Excess water should also be avoided because it can reduce coverage. Thus, option B is correct.