A black outline is ineffective inside a very dark poster colour area. What is a better option?

Correct answer: Use a light contrasting coloured line

The governing concept is value contrast and visual legibility. A black outline placed inside a very dark area has nearly the same lightness value as its background, so the boundary disappears. A light contrasting line, such as pale yellow or white, creates a stronger value difference and makes the form readable. Increasing the thickness of black does not create the needed contrast, while removing or diluting the area does not solve the outline problem. Therefore option A is correct.