Which brush should be chosen for filling small areas?

Correct answer: Thin brush

The governing concept is selecting a brush according to the size and detail of the area. A thin brush has a smaller tip, so it gives better control near outlines and in narrow spaces; therefore, option B is correct. A wide brush covers large regions but may cross boundaries, while a cloth or roller cannot provide the precision needed for small painted sections. The artist should also match the brush shape and moisture to the required edge and finish.