What is the correct way to create a thick-to-thin line in one stroke?

Correct answer: Gradually reduce pressure

The governing concept is pressure control during a continuous brush stroke. Begin with slightly greater pressure so that more bristles contact the surface and make a thicker mark, then gradually reduce pressure while moving forward. Fewer bristles touch the surface, so the line tapers to a thinner end. Stopping creates a blot, adding water changes paint consistency rather than reliably controlling width, and changing the handle is irrelevant. Thus option A is correct.