Which map feature uses lines to show relief by varying thickness, indicating slopes?

Answer: A — Hachures

Lines that depict relief by thickness are called hachures. They are short strokes drawn on the slope of the terrain, and their length, density, and thickness convey how steep a surface is—the more and longer the strokes, the steeper the slope; on gentler areas the lines are sparser and often shorter. This line work provides a quick sense of three-dimensional shape using variation in a few strokes. By contrast, contour lines are simply lines of equal elevation, isobars map atmospheric pressure, and shading relies on tonal gradations rather than line thickness to imply relief. So this feature uses lines whose thickness and density communicate slope.