Which of the following best describes AC voltage?

The definition of AC (Alternating Current) voltage is accurately described by the statement that it alternates polarity periodically. This characteristic of AC voltage means that the voltage levels fluctuate over time, reversing their direction at regular intervals. The most common example of AC voltage is the electricity supplied to homes and businesses, which typically varies between positive and negative values in a sinusoidal waveform.

In contrast, options describing voltage that remains constant over time pertain to DC (Direct Current) voltage, where the electricity flows in a single, consistent direction. Additionally, while batteries indeed provide a steady supply of voltage, this corresponds to DC systems rather than AC. The mention of voltage being used exclusively in digital circuits is misleading, as digital circuits can operate on both AC and DC voltages depending on the specific design and application.