The correct option is Rheostat.
Explanation
According to Ohm’s Law ($I = V/R$), the current ($I$) flowing through a conductor is inversely proportional to its resistance ($R$) when the potential difference ($V$) remains constant. To regulate or vary the current in a circuit without altering the voltage source, the resistance of the circuit must be adjusted. A component designed to provide variable resistance is required for this purpose.
Option Analysis
- Rheostat Rheostat: Correct. A rheostat is a variable resistor used to control the flow of electric current by manually increasing or decreasing the resistance. It is connected in series with the circuit to adjust the current magnitude.
- Voltmeter Voltmeter: Incorrect. A voltmeter is an instrument used to measure the electrical potential difference between two points in an electric circuit. It has very high resistance and is connected in parallel; it does not regulate current.
- Battery Battery: Incorrect. A battery acts as a source of electromotive force (EMF) or voltage. While it drives the current, it is not a control device used to vary current magnitude within an existing circuit configuration.
- Diode Diode: Incorrect. A diode is a semiconductor device that primarily allows current to flow in one direction (rectification). While it affects current flow, its primary function is not the regulation of current magnitude via variable resistance.
Key Takeaway: A rheostat functions as a variable resistor, allowing the user to alter the resistance in a circuit to control the current intensity without changing the voltage source.