What property of an AC signal distinguishes it from a DC signal?

The property that distinguishes an alternating current (AC) signal from a direct current (DC) signal is that AC signals can reverse direction periodically. This characteristic is fundamental to AC, which varies in amplitude and direction over time, typically following a sinusoidal waveform. As a result, the current flows in one direction for part of the time and then reverses to flow in the opposite direction. This periodic change in direction is what allows AC to be effectively transformed and transmitted over long distances, making it essential for powering homes and industries.

In contrast, DC signals maintain a constant magnitude and direction, providing a steady flow of electricity. This does not occur in AC signals. Other options suggest inaccuracies about AC; for example, AC signals indeed have frequency characteristics, which are essential for determining how they behave in circuits and their compatibility with different electrical devices. Additionally, resistance is not inherently zero in AC; rather, circuits may have specific impedance based on frequency and load. Thus, the ability of AC signals to reverse direction is what fundamentally sets them apart from DC signals.