ARRL Technician Practice Exam 2025 - Free Technician License Practice Questions and Study Guide

The correct answer is the component known as a variable resistor. A variable resistor is specifically designed to provide resistance that can be adjusted as needed. It often takes the form of a potentiometer or a rheostat, allowing users to change the resistance value in a circuit. This ability to change resistance can be crucial for controlling various parameters such as voltage, current, or power levels in a circuit.

In contrast, a transformer is used to change the voltage level in alternating current (AC) circuits without changing the frequency, but it does not provide variable resistance. A capacitor is a component that stores electrical energy temporarily and is commonly involved in filtering and timing applications but does not serve the purpose of varying resistance in a circuit. Similarly, a diode is used to control the direction of current flow, allowing current to pass in one direction only, but it does not function as a resistor of any type, variable or otherwise. Therefore, the variable resistor is the only component listed that fulfills the role of being adjustable within the context of resistance in a circuit.