Why is it useful to have a clear boundary before filling a shape with colour?

Correct answer: It helps keep the colour within the intended area

A clear, continuous boundary defines the area that should receive paint and gives the artist a visual guide while working near the edges. Thus option A is correct because it helps keep colour inside the intended shape and produces a neat result. An outline does not automatically increase brightness, speed the drying of every medium, or alter the physical shape of a brush; those claims describe unrelated effects.