Alternating current (AC) is best described as a current that alternates between:

Alternating current (AC) is best described as a current that alternates between positive and negative polarity. This characteristic means that the flow of electric charge periodically reverses direction, which is a defining feature of AC. In practical terms, AC voltage alternates in polarity, causing the electrons to flow in one direction and then reverse to flow the other way. This alternating behavior is utilized in many applications, especially in power systems, as it allows for efficient transmission of electricity over long distances.

In contrast, other options do not accurately capture the fundamental nature of AC. High and low voltage or high and low amperage refer to the magnitude of electrical measurements rather than the directional changes inherent to AC. Similarly, while frequency is a relevant concept with AC (indicating how many times the current alternates direction per second), it does not directly describe the alternating characteristic itself. Polarity, on the other hand, specifically addresses the positive and negative aspects of the current flow, making it the most appropriate description of AC.