The correct answer is: D. Resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors are used to reduce current flow, to create voltage dividers, to bias active elements, to terminate transmission lines, and to adjust the impedance of a circuit.
A reactor is an electrical component that stores energy in a magnetic field. Reactors are used in AC circuits to provide inductance, which opposes changes in current.
A capacitor is an electrical component that stores energy in an electric field. Capacitors are used in AC circuits to provide capacitance, which opposes changes in voltage.
Inductors and capacitors can be used to reduce current in AC circuits, but they are not as effective as resistors. This is because inductors and capacitors only oppose changes in current, while resistors oppose current flow regardless of whether the current is changing or not.
In conclusion, the most effective way to reduce current in a circuit is to use a resistor.