What device is primarily used to protect electrical circuits from overheating?

Fuses are designed specifically to protect electrical circuits from overheating and potential damage caused by excessive current flow. They operate on the principle of melting when the current exceeds a predetermined level, thus interrupting the circuit and preventing further electrical flow. This action mitigates the risk of overheating, which can lead to component failure, fires, or other hazardous conditions.

Thermocouples are used for temperature measurement rather than circuit protection. Shunts are employed to bypass current and measure the flow, but they do not serve a protective function. Resistors manage the flow of current in circuits but do not disconnect the circuit when overheating occurs. Therefore, fuses are the most appropriate answer as they fulfill the critical role of safeguarding electrical circuits from overheating.