_______ is an on-off device for current (or an appliance). Fill in the blank.
A. MCB
B. Switch
C. Socket
D.Plug

Hint: To answer the above question, think that a device we use in our home and classroom very frequently to on-off our fans and lights, this is a plastic device and acts as an insulator to keep us safe from electric shocks.

Complete Step By Step Answer
MCB: MCB stands for Miniature Circuit Breaker. MCB is used for safety purposes. It is an electromagnetic device used to control overflow of current in the circuit, it replaces fuse in most of the circuits, as always we need to replace fuse after the overflow of current, MCB replaced fuse because MCB breaks the circuit when it finds overflow current running in the circuit. It also detects a problem in the circuit and gets down automatically, from which we get an indication that there is any problem in the circuit.
Switch: Switch is an electric appliance used to on-off the device for current in a circuit. The switch is always connected with the live wire. It is also an insulator.
Socket: It is used to connect with electric plugs. It contains two or three hollow parts where two-pin or three-pin plugs get fits. Fitted Plugs and sockets connect the electric appliance and can pass electricity if their respective switches are on.
Plug: A plug is a small plastic electrical equipment containing two or three metal pins that fit in two or three-hole electric sockets and connect the electric appliance and can pass electricity if their respective switches are on.
From the above information, you will definitely get to know that switch is an on-off device for current.

Note: Point to note is, as per the above information you get to know that MCB is also a device that is used on-off, but you keep in the mind that MCB is used to control the whole circuit which contains many switches and devices.