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

The correct answer is a unit of an electrical system that carries or controls electrical energy as its principal function. A device encompasses a broad category and can include various components that perform specific functions, such as switches, resistors, or integrated circuits, which do not necessarily carry electrical energy as their primary role.

In contrast, a conductor is specifically meant to carry electrical energy, such as wires or cables. A transformer is a device that modifies electrical energy levels through electromagnetic induction. A connector, while important in establishing electrical connections, does not carry or control energy as its primary function but rather serves as a point of connection between various components.

Thus, while the term "device" might describe many items within an electrical system, it lacks the specificity needed to identify a component whose main role is to carry or control electrical energy. The focus on carrying or controlling energy points more directly to the precise nature of conductors and transformers, making the correct term a more accurate designation.