Which brush is most suitable for writing small letters with poster colour?

Correct answer: Thin pointed brush

The governing concept is choosing a tool according to the scale and precision of the work. A thin pointed brush holds a manageable amount of colour and produces narrow, controlled strokes, so option C is correct for small lettering. A very broad flat brush is better for large areas, a sponge cannot form precise letter lines, and a wire brush is unsuitable because it can damage the paper and cannot carry paint smoothly.