What is the purpose of a relay in an electrical circuit?

The purpose of a relay in an electrical circuit is to allow a small current to control a larger current. A relay functions as an electromechanical switch; when a small current passes through its coil, it generates a magnetic field that activates a switch mechanism. This mechanism either opens or closes, allowing or interrupting the flow of a larger current in another part of the circuit. This capability is crucial in many applications where it is impractical or unsafe to carry high currents through the control switches directly, providing an efficient way to manage various electrical loads safely and effectively.

The other options are less relevant to the fundamental role of a relay. Measuring voltage pertains to voltage meters and measurement devices, while increasing resistance is associated with resistors, which do not involve the switching functionality of relays. Preventing current flow might describe a switch in the "off" position, but it does not encapsulate the primary function of a relay, which is to facilitate control of current, not simply to block it.