Which of the following is a type of signal that varies over time in an alternating fashion?

The correct answer is AC signal, which stands for alternating current signal. An AC signal is characterized by its varying voltage or current over time, typically in a sinusoidal wave pattern. This means that the signal oscillates above and below a central value (usually zero), creating a periodic waveform. This variability is fundamental to AC signals and is what distinguishes them from other types of signals.

While a digital signal consists of discrete levels, often representing binary values of 0 and 1, it does not vary continuously over time in the same manner as an AC signal does. Analog signals, on the other hand, can also vary over time and represent a continuous range of values, but they do not specifically indicate the nature of the oscillation as AC signals do. A static signal remains constant and does not vary, which is opposite to what an AC signal represents. Hence, the identifying characteristic of an AC signal in this context is its alternating, time-varying nature.