Which brush is useful for filling a large area with acrylic colour?

Correct answer: Broad flat brush

The governing tool-selection principle is to match brush shape and size to the area being painted. A broad flat brush carries enough paint and covers a wide region with fewer, more even strokes, so option C is correct. A very thin brush is better for lines and details, a wire brush is not a normal paintbrush for smooth coverage, and a brush without bristles cannot apply paint effectively.