Which brush is more useful for painting small flower petals with poster colour?

Correct answer: Thin round brush

The governing tool-selection principle is to match brush size and shape with the detail being painted. A thin round brush has a pointed tip and flexible hairs, allowing the artist to make narrow curved strokes for small petals. Thus option A is correct. A broad flat brush is intended for larger areas, while a wire brush or a brush without bristles cannot produce controlled, smooth poster-colour details.