The correct answer is C) remotely open and close contacts by action of a coil. An electromagnetic relay is a device that uses an electromagnetically operated switch to open or close electrical contacts remotely. When current flows through the coil, it creates a magnetic field that pulls the switch contacts together, closing the circuit. This allows a small electrical signal to control a much larger electrical load, which is the key purpose of a relay. The other answer choices are incorrect as they do not accurately describe the primary function of an electromagnetic relay.