Which of the following best describes alternating current?

Alternating current (AC) is defined as a flow of electric charge that regularly reverses its direction. This means that in an AC circuit, the movement of electrons shifts back and forth, changing direction periodically, typically in a sinusoidal manner. This property allows AC to be easily transformed to different voltages, which is advantageous for long-distance power transmission, and is commonly used in household electricity supply.

The other options do not accurately describe AC. For example, a flow of electric charge that steadies in one direction characterizes direct current (DC), where the electric charge flows consistently in a single direction. Variability in current, as mentioned in another option, could suggest a fluctuating current but does not specifically capture the primary characteristic of AC, which is the reversal of direction. Lastly, the option about a flow of electric charge that decreases over time does not pertain to the behavior of AC, as it does not inherently decrease but rather oscillates between positive and negative values. Hence, the concept of reversal is the quintessential feature that defines alternating current accurately.