Which term defines "a unit of an electrical system that carries or controls electric energy as its principal function"?

The term that defines "a unit of an electrical system that carries or controls electric energy as its principal function" is "device." In electrical terminology, a device refers to any apparatus that is capable of performing a specific function, particularly in the regulation, control, or distribution of electrical energy. This includes items like switches, circuit breakers, and relays, which are integral parts of electrical systems designed to manage the flow of electricity.

While the other options also relate to electrical functions, they do not specifically emphasize the primary role of carrying or controlling electric energy. An appliance generally refers to a piece of equipment designed for a specific task, often in the context of home or industrial use, rather than to a unit specifically focused on electrical control. A system component is a broader term that may include any part of a system, whether it directly carries or controls electrical energy or not. Equipment usually refers to a collection of devices or tools necessary for a specific function or task, without highlighting the electricity management aspect as the main function. Thus, "device" is the most precise term in this context.