An Alternating Voltage (AC) is defined as a voltage that varies in magnitude with time and reverses its polarity (direction) periodically . While the most common form of AC discussed in the textbook is sinusoidal , any waveform (such as square, triangular, or complex waves) that alternates between positive and negative values is technically an AC voltage. Therefore, if all the given figures (a, b, c, d) show the EMF changing polarity, they are all AC voltages.