What visual cue indicates the presence of rust on carbon steel?

The presence of rust on carbon steel is visually identified by a reddish-brown color. This discoloration occurs when iron, the primary component of carbon steel, reacts with oxygen and moisture in the environment, leading to the formation of iron oxide, commonly known as rust. The reddish-brown hue is a clear and distinct marker of corrosion, making it easy to identify the affected areas on the steel surface.

The other options do not accurately represent rust. A shiny metallic appearance indicates a clean, uncorroded surface, while clear transparency does not correlate to any rust condition, as rust obscures visibility. Dark gray residue could suggest other forms of corrosion or deposits, but it is not characteristic of rust on carbon steel. Thus, the reddish-brown color is the definitive visual cue for identifying rust.