A transformer is both a ______ and a _______.

The correct choice highlights that a transformer serves as both a load and a source in an electrical system.

A transformer functions by transferring electrical energy between circuits through electromagnetic induction. When it is connected to an AC power supply, the transformer acts as a load, drawing power from the source. This is particularly evident when the transformer is in a step-down configuration, reducing voltage and delivering it to a lower voltage circuit.

Simultaneously, the transformer also acts as a source. It provides transformed electrical output to the circuit it feeds, supplying power at the desired voltage and current levels required by that circuit. This dual functionality is critical in various applications, including power transmission and distribution systems, where transformers ensure that electricity can be modified to meet specific voltage needs.

The other options do not accurately define the role of a transformer within electrical systems, as they misrepresent the fundamental operation and characteristics of transformers. For example, while a transformer is a device, not all devices are sources or loads in the same context, and terms like 'switch' or 'relay' describe different functions entirely.