The waveform of alternating current changes from positive to negative. What does this indicate about the current flow?

The correct choice indicates that the alternating current (AC) waveform is not steady or static; it changes direction and magnitude over time, showcasing its fluctuating nature. In AC, the waveform periodically transitions from positive values, representing one direction of current flow, to negative values, indicating the opposite direction. This back-and-forth movement contributes to the fluctuating characteristic of AC, where the current continuously varies in both direction and intensity.

The other choices do not capture this essence of AC accurately. The notion of constant implies that the current flows uniformly in one direction without change, which is not true for AC. The term variable could suggest it changes but lacks the precision to define the cyclical nature of AC. Lastly, unidirectional specifically refers to direct current (DC), where the current flows in only one direction, contrasting with the inherent alternating behavior of AC.