How should brush-stroke direction be adjusted when the light direction changes on a round object?

Correct answer: Follow the curvature while changing tone according to light

The governing concept is descriptive brushwork: stroke direction explains the rounded form, while tonal value explains where light and shadow fall. Thus, strokes should generally follow the object’s curvature, but their darkness and lightness must respond to the changing illumination. The other options ignore form, confuse light direction with stroke direction, or impose straight lines that flatten a round object.