The correct option is Rheostat.
Explanation
In a direct current (DC) circuit, the flow of current is governed by Ohm’s Law ($I = V/R$), where $I$ is current, $V$ is voltage, and $R$ is resistance. To vary the current without changing the voltage source, the total resistance of the circuit must be adjustable.
Option Analysis:
- Capacitor Capacitor: Incorrect. A capacitor stores electrical energy in an electric field. In a DC circuit, it blocks steady current once it is fully charged, acting as an open circuit rather than a current regulator.
- Rheostat Rheostat: Correct. A rheostat is a variable resistor usually consisting of a resistive wire and a sliding contact. By adjusting the position of the slider, the length of the wire included in the circuit changes, which alters the resistance. Changing the resistance allows the user to control the magnitude of the current flowing through the circuit while the source voltage remains constant.
- Diode Diode: Incorrect. A diode is a semiconductor device that allows current to flow primarily in one direction. It is used for rectification or as a switch, not for continuously varying current magnitude.
- Transformer Transformer: Incorrect. A transformer works on the principle of mutual induction, which requires a changing magnetic flux. Consequently, it operates only with Alternating Current (AC) and does not function in a steady DC circuit.
Key Takeaway: A rheostat is a device used to vary the resistance in a circuit, thereby controlling the current intensity according to Ohm's Law, without the need to change the voltage source.