What is a relay?

• A. A resistor network
• B. A transistor amplifier
• C. A mechanical switch
• D. An electromagnetic device activated by current in one circuit to open or close another circuit

Answer: (D)

A relay is an electromagnetic switch that uses current in one circuit to control a switch in another circuit. When the control coil is energized, it creates a magnetic field that moves the relay’s contacts, opening or closing the load circuit. This lets a small, low-power signal control a larger current in a separate path and provides isolation between the control and load sides.

That description fits exactly: an electromagnetic device activated by current in one circuit to open or close another circuit. A resistor network is just passive resistance, a transistor amplifier changes signal strength without switching a separate circuit, and a mechanical switch is typically operated manually rather than by current in a different circuit.