Identify the device used to protect circuits from overvoltage.

A surge protector is specifically designed to protect electrical devices and circuits from overvoltage conditions, which can occur due to events such as lightning strikes, power surges, or sudden changes in electricity demand. This device works by diverting excess voltage away from sensitive components and limiting the voltage that enters the protected equipment. By doing so, it helps prevent damage caused by power spikes, thereby ensuring the longevity and functionality of the connected devices.

In contrast, a fuse is primarily used to protect circuits from overcurrent rather than overvoltage. A transformer adjusts voltage levels for power distribution but does not provide protection against surges. A resistor is a component used to limit current flow or provide voltage division but does not offer any protective capabilities against overvoltage. Thus, the surge protector stands out as the right option for safeguarding circuits from excessive voltage levels.