Which pigment bias is more suitable for obtaining a clean violet in poster colour?

Correct answer: Blue-biased red and red-biased blue

The governing concept is subtractive pigment bias. To mix a clean violet, choose a red that leans toward blue and a blue that leans toward red; these pigments introduce less unwanted yellow or green influence into the mixture. Thus B is correct. Yellow-biased red and green-biased blue tend to produce a duller, contaminated violet, while orange–green and yellow–black are not the appropriate primary-pigment pair for a clear violet.