A common symptom of voltage drop is _____________________.

A common symptom of voltage drop is that lights become dim. This occurs because when there is a significant drop in voltage within a circuit, the amount of electrical energy available to power devices, such as lights, diminishes. Electrical devices, including lights, require a certain voltage to operate effectively; as the voltage decreases, the brightness of incandescent bulbs, for instance, can be noticeably reduced. In the case of LED lighting, while they might not dim in the same way, a significant voltage drop can still negatively affect their performance and lifespan.

The other options relate to different electrical or operational conditions that do not directly indicate a voltage drop. For example, fan motors running fast could suggest an issue with electrical supply or control rather than a reduction. An air conditioning unit blowing colder than normal might indicate efficient operation or could be affected by other system factors, but it is not a reliable indicator of voltage drop. A backup generator starting up typically indicates that a power supply issue is present, but this is more a response to an outage or high demand rather than a sign of voltage drop itself.