What item is primarily used to prevent circuit overload?

The item primarily used to prevent circuit overload is the fuse. A fuse is designed to protect electrical circuits by breaking the circuit when the current exceeds a predetermined level. When an overload occurs, the fuse element melts, thereby opening the circuit and stopping the flow of electricity. This is a critical safety feature, as it helps to prevent potential damage to the electrical system and reduces the risk of fire hazards.

While a transformer can step up or step down voltage levels and a battery stores energy for a circuit, neither of these components is specifically used for overload protection. A capacitor, on the other hand, is used for storing electrical energy and smoothing out electrical signals but does not perform a protective function against overloads. Therefore, the fuse is the correct choice because its primary purpose is to prevent damage from excessive current by interrupting the power supply.