What symptom may indicate a failing alternator?

The correct answer is related to the electrical system of the vehicle. Dim headlights or dashboard lights can indicate a failing alternator because the alternator is responsible for charging the battery and powering the electrical systems while the engine is running. If the alternator is not functioning correctly, it may not provide sufficient voltage to keep the lights bright. This can be particularly noticeable when the engine is idling or under low load conditions, where the reduced electrical output from a failing alternator would lead to dim or insufficient lighting.

In contrast, the other symptoms listed are linked to different issues. For example, a grinding noise during engine startup usually points to problems with the starter or related components. Rough idling could indicate issues with fuel delivery, ignition timing, or vacuum leaks, which are not directly related to the alternator's function. Increased fuel consumption is typically associated with problems in the engine or the fuel system rather than electrical concerns. Therefore, the dimming of lights is a clear and direct symptom of potential alternator failure, making it the correct choice.