Which device is capable of supplying and maintaining voltage while some type of electrical apparatus is connected to its terminals?

A voltage source is specifically designed to supply and maintain a constant voltage level to an electrical circuit or device connected to its terminals. It provides the necessary electrical energy to power devices while ensuring that the voltage supplied remains stable, even as the load changes. This characteristic is foundational in electronics, making voltage sources critical components in power supplies, batteries, and similar devices.

In contrast, a current regulator primarily focuses on maintaining a specific current flow rather than voltage levels. A resistance unit is not a source of power; instead, it limits or modifies the flow of electric current through its inherent resistance. Lastly, an electrical load represents components or devices that consume electrical energy, rather than providing or regulating it. Thus, the definition and functionality of a voltage source align perfectly with the requirements of supplying and maintaining voltage in a connected circuit.