Which brush is useful for lettering in a poster painting?

Correct answer: Thin or flat controlled brush

The governing concept is brush control: lettering needs a tool that can produce deliberate strokes, clean edges, and reasonably consistent thickness. A thin brush is useful for fine lines, while a small flat brush can fill letter shapes neatly. Therefore, option D is correct. A broom, cotton bundle, or broken pencil cannot reliably control wet poster colour or form clean painted letters.