What type of device might be used to protect a circuit from overload?

A fuse is specifically designed to protect a circuit from overload conditions. It functions by serving as a safety device that interrupts the circuit when the current flow exceeds a predetermined limit. This is accomplished through a thin wire inside the fuse that melts when the current is too high, effectively breaking the circuit and preventing damage to other components.

The other devices listed do not serve this protective function. For instance, a capacitor is primarily used for storing and releasing electrical energy, while a transformer is involved in changing voltage levels in an AC circuit. A diode allows current to flow in one direction only, providing rectification, but does not interrupt the circuit in case of overload. Therefore, the fuse is the most suitable option for protecting a circuit from overload conditions.