What component is specifically designed to melt when a certain temperature is reached?

The component designed to melt at a specific temperature is the fuse. Fuses are safety devices that protect electrical circuits from overcurrent conditions that could lead to overheating and potential fires. They contain a wire or metal strip that melts and breaks the circuit when the current exceeds a predetermined level for a certain amount of time. This melting action interrupts the flow of electricity and prevents damage to the circuit and connected devices.

Fuses operate on the principle of thermal overload; when too much current flows through the fuse, it generates heat. Once the heat reaches a critical temperature, the conductive material within the fuse melts. This destruction of the conductive pathway is what makes fuses effective at preventing more severe electrical faults.

In contrast, other components listed serve different functions. Resistors are used to limit current or divide voltages, but they do not melt in response to temperature and are not designed for protection against overloads. Circuit breakers offer a resettable method of protection, tripping in response to overcurrent but not melting; they can be reset once the fault is resolved. Insulators are materials that prevent the flow of electricity but are not designed to act as protective devices in current overload scenarios. Hence, the fuse stands out as the correct answer to this question.