Which brush is useful for lettering with acrylic colour?

Correct answer: A suitable flat or pointed brush

The governing concept is selecting a brush according to the visual task. Lettering needs controlled strokes, clean edges, and a tip or edge suited to the size and style of the letters. A suitable flat brush can make broad, even strokes, while a pointed brush can form narrower details. Thus option D is correct. A wire brush or bristle-free brush cannot apply paint properly, and a very broad brush is not suitable for every letter size.