____________is a device used for varying the resistance of an electrical circuit.

Correct option is D

The correct answer is (D)   ​Rheostat

​A rheostat is a device used to adjust the resistance in an electrical circuit, often called a variable resistor.

• A rheostat has two terminals and a movable contact.
• By sliding or turning a knob, the effective length of the resistor changes, altering the resistance.
• Increasing resistance reduces the current, while decreasing resistance allows more current to flow.
• Controlling the flow of electric current in circuits.
• Adjusting generator characteristics.
• Dimming lights.
• Regulating the speed of electric motors.

Information booster:

Ammeter

An ammeter measures the current flowing through a circuit.

• Connected in series with the circuit.
• It has low resistance to ensure it does not affect the current.

​Galvanometer

A galvanometer is a device used to detect small electric currents in a circuit.

• It is highly sensitive and measures the direction of current.
• Used in experiments to detect and measure tiny currents.

​Voltmeter

A voltmeter measures the potential difference (voltage) between two points in a circuit.

• Connected in parallel to the circuit.
• It has high resistance to avoid altering the current flow.