Which brush is more useful for making small dots with poster colour?

Correct answer: Thin round brush

Brush shape controls the size and precision of a mark. A thin round brush has a fine point that can hold a small amount of poster colour and place it accurately, making option A correct for small dots. A broad flat brush creates wider marks, a wall roller is intended for large surfaces, and a dry sponge produces a textured patch rather than a precise dot.