Which option shows contrast in line thickness?

Correct answer: A thick line beside a thin line

Contrast is created when two visual qualities differ clearly. In option A, a thick line is placed beside a thin line, so the measurable quality being compared—line thickness—changes from large to small. That makes A the correct answer. Options B, C, and D retain the same thickness; their curvature or repetition may create other effects, but they do not demonstrate thickness contrast.