What is a unit of an electrical system that carries or controls electrical energy as its principal function?

The concept behind the correct answer revolves around the definition of a 'device' in an electrical system. A device is broadly understood as any unit that performs a specific function, particularly in controlling or managing electrical energy. This can include a range of components, from resistors to complex systems like circuit breakers or switches, which actively manage the flow of electricity.

In this context, while a conductor refers specifically to materials or wires that allow electrical current to pass, it does not control energy in the way a device does. Similarly, a relay functions as a switch that opens or closes circuits but is a specific type of device rather than a general term. A transformer transforms voltage levels in an electrical system but does not inherently carry or control energy as its principal function in the broader sense.

Choosing 'device' emphasizes the broader role these units play within electrical systems, highlighting their integral part in managing and controlling electrical energy. This term encompasses various types of mechanisms necessary for the smooth operation of an electrical system, leading to the conclusion that it aligns best with the question's criteria.