When current flows in circuits that include alternating current, which of the following is true?

The flow of current in alternating current (AC) circuits is characterized by its periodic reversal of direction. This means that, unlike direct current (DC), where the flow of electrons moves in a single, unidirectional path, AC current changes its direction and magnitude over time in a sinusoidal manner. This is a fundamental trait of AC, and the frequency at which this direction change occurs is measured in hertz (Hz).

The truth in this context is that AC is designed for specific applications, such as powering homes and appliances, where such periodic reversal is beneficial for the efficient transmission of electricity over distances. This characteristic allows for the use of transformers and various devices to manage voltage levels and improve distribution.

In contrast, constant or unidirectional flow, indicated by other options, pertains to DC circuits where the current does not reverse. The concept of instability does not accurately describe the regulated nature of AC systems, which are carefully engineered to maintain predictable oscillations. Overall, the periodic reversal of current direction is the defining feature of alternating current systems.