Which brush is more useful for making a thin long continuous line?

Correct answer: A liner brush with long bristles

The governing concept is matching a brush shape to the mark it produces. A liner brush has a narrow point and long bristles that can hold enough paint for a steady, uninterrupted stroke. Therefore option A is correct. A sponge brush makes broad textured marks, a stippling brush produces dots, and a wall brush is intended for wide coverage rather than delicate lines.