Which brush is suitable for filling a large area with poster colour?

Correct answer: Broad flat brush

The governing technique is matching brush size and shape to the area being painted. A broad flat brush carries enough poster colour and covers a large surface evenly with fewer strokes, so option A is correct. A thin round brush and a liner brush are better for narrow lines and small details. A fan brush is mainly useful for texture or soft effects, not for laying an even, solid coat over a large area. The brush should be loaded moderately to avoid drips.