Which of the following best describes 'alternating current'?

Alternating current is characterized by the periodic change in direction of flow. This means that the electric charge shifts back and forth rather than maintaining a constant flow in one direction, as seen in direct current. The ability of alternating current to change direction is beneficial for various applications, including its effectiveness in power distribution systems. It allows for easier transmission of electricity over long distances and can be transformed to different voltage levels to improve efficiency.

The other options misrepresent alternating current; for example, constant current that only flows in one direction refers to direct current, while suggesting that alternating current is exclusive to mobile devices or limited to residential areas does not accurately reflect its broad usage in different sectors, including industrial and commercial applications.