How can lines of different thicknesses be made with the same brush?

Correct answer: By changing pressure and angle

The governing concept is controlling brush contact with the surface. Pressing more of the bristles onto the paper generally creates a broader line, while lighter pressure or a different angle can create a narrower line. Therefore option B is correct. Changing only the colour changes hue, not line thickness, and changing paper or cutting the handle is not a normal brush-control method.