What type of voltage varies continuously with time?

The choice of analog voltage as the correct answer stems from its defining characteristic of varying continuously over time. In an analog voltage signal, values can change smoothly and can represent a range of values, which makes them suitable for applications needing precise and gradual variations.

For instance, an analog voltage signal can take on any value within a certain range, such as from 0 to 5 volts, and it can transition through all values in between without any jumps or discrete steps. This property is key in many applications, including audio signals, where sound waves are represented as continuous voltage variations.

While AC (Alternating Current) voltage also varies continuously and is often confused with analog voltage, the term specifically relates to how a signal is processed or represented rather than just how the voltage behaves. AC voltage, for example, alternates between positive and negative values in a sinusoidal manner, but it's primarily related to electrical power transmission.

Digital voltage, on the other hand, typically represents two discrete states, commonly identified as '0' and '1'. This means that digital voltage is not continuous, as it only takes on specific values rather than a continuous range.

DC (Direct Current) voltage, while constant over time (once the steady state is reached), does not