Which brush is most suitable for making a thin line?

Correct answer: Round pointed brush

The governing concept is that brush shape determines the kind of mark it can make. A round pointed brush has a tapered tip that holds paint and releases it in a narrow, controlled path, making it suitable for thin lines. Therefore option A is correct. A wide flat brush is designed for broad areas, while sponge and roller brushes generally produce wider or textured coverage rather than a precise fine line.