Which device is the smallest and is capable of amplifying electrical signals?

The smallest device capable of amplifying electrical signals among the options is indeed a relay. A relay is an electromechanical switch that uses an electromagnetic coil to control a larger current in a circuit, effectively amplifying the control signal that operates it.

When a small current flows through the coil, it creates a magnetic field that activates the switch, allowing a larger current to pass through a different circuit. This amplification process can control high-power devices with a lower-power signal, making relays essential in various electronic applications.

While capacitors, diodes, and transformers have important roles in electrical circuits—such as storing energy, allowing current to flow in one direction, and changing voltage levels, respectively—they do not serve the same purpose of amplifying signals in the way relays do. Understanding the functionalities of each device is crucial in electronics as it helps in determining their suitable applications in circuits.