What can be done to make a hard boundary between dark and light poster-colour areas appear slightly softer?

Correct answer: Create a controlled middle value and apply it in dry layers

The governing concept is a value transition. A middle value placed between the dark and light areas reduces the abrupt change and makes the boundary appear visually softer. In poster colour, applying that transition in controlled, dry layers preserves opacity and prevents uncontrolled bleeding, so option C is correct. Excess water may cause stains, black makes the edge harsher, and rubbing can damage the paper or lift paint.