What may happen if lettering is painted over a wet poster-colour background?

Correct answer: The edges of the letters may spread

The governing concept is wet-on-wet behaviour. A wet poster-colour background can partially mix with the lettering paint, allowing the new colour to travel into the damp layer; as a result, letter edges may spread, soften, or blur. This reduces legibility rather than making the letters clearer. Wet paint does not become instantly permanent, and moisture does not make the paper thicker. Letting the background dry before lettering gives sharper, more controlled outlines, so option D is correct.