Which device is designed to protect a circuit against current overloads?

A fuse is specifically designed to protect electrical circuits from current overloads. It does so by containing a metal strip or filament that melts and breaks the circuit when the current exceeds a certain threshold. This action prevents excessive current from flowing through the circuit, which can cause damage to the components, overheating, or even fires. The fuse acts as a safety device, ensuring that when there is an overload, the circuit is interrupted, thereby safeguarding the entire system.

The other devices mentioned serve different purposes: a transistor is used for amplifying or switching electronic signals, a relay is an electromechanical switch that opens or closes circuits based on an electrical signal, and a capacitor stores electrical energy temporarily but does not protect against current overloads. Thus, the fuse is uniquely suited to provide the necessary protection against excessive current, making it the correct answer.