Which is the most controlled way to lighten the value of a poster colour?

Correct answer: Add white in small amounts

The governing concept is gradual value adjustment. White raises the lightness of a poster colour, but adding it in small portions allows the artist to observe the change and stop at the desired value. Hence, option B is correct. A large amount may make the mixture too pale; water mainly changes transparency and flow, while adding black and white together can produce an unwanted dull or grey result.