Which of the following best characterizes a relay's operation?

A relay is essentially an electromechanical switch that uses an electromagnet to control the switching of circuits. When an electric current passes through the coil of the electromagnet, it generates a magnetic field that attracts a movable armature. This movement either opens or closes the contacts in the relay, thereby turning the circuit on or off. This ability to control larger power circuits with a small control signal is a fundamental characteristic of relays, making them invaluable in automation and control applications.

The other options are less representative of a relay's main function. For instance, altering the frequency of power signals is typically associated with devices such as oscillators or frequency converters, not relays. Similarly, while there are solid-state relays that operate without mechanical movement, traditional relays do involve a mechanical component with the movement of the armature. Measuring electrical resistance is a function of multimeters or ohmmeters, which have no relation to the switching operation of relays. Thus, the correct characterization of a relay's operation is its use of an electromagnet to control circuit switching.