Which brush is useful for maintaining uniform thickness in letters?

Correct answer: A flat brush of suitable size

The governing lettering concept is controlled, even stroke width. A flat brush of suitable size has a straight edge and a consistent breadth, so it can deposit paint across a letter stroke with relatively uniform thickness. A round brush changes width more readily with pressure or angle, while a sponge and a worn brush cannot make clean, controlled edges. Therefore option B is correct.