When it is necessary to switch on an electrical current from a remote location, which device could be used?

A relay is the appropriate device for switching on an electrical current from a remote location. Relays act as electrically operated switches, allowing a low-power signal to control a larger power circuit. When a current flows through the relay's coil, it creates a magnetic field that closes or opens the contacts, enabling the control of electrical devices or circuits from a distance. This functionality makes relays ideal for applications where it is impractical or unsafe to operate a switch directly, such as in various automated systems, remote control mechanisms, and safety circuits.

The other devices listed serve different functions: a diode regulates current flow in one direction, an amplifier boosts signals often in audio or radio applications, and an oscillator generates periodic waveforms. None of these devices have the capability to switch current on or off from a remote location in the same manner as a relay.