What kind of current does an alternating current (AC) supply?

An alternating current (AC) supply is defined as a type of electrical current that periodically reverses its direction of flow. This means that the voltage level alternates between positive and negative over time, allowing the current to flow in one direction during part of the cycle and then in the opposite direction during another part. This alternating nature is typically measured in cycles per second, known as Hertz (Hz).

In household and commercial power systems, AC is preferred because it can be easily transformed to different voltage levels for efficient long-distance transmission and distribution. The ability to change direction also enables the use of AC in various applications, such as electric motors and generators, which operate based on this property of alternating flow.

Regarding the other options, the description implying a constant current or a current that flows only in one direction does not apply to AC. Additionally, while a closed circuit is necessary for current to exist and flow, this characteristic is not unique to AC; it applies to all electrical currents, including direct current (DC). Thus, the distinctive attribute of AC is its periodic reversal of direction, which is accurately captured by the correct choice.