What type of electrical device is primarily used for changing the voltage level in an AC system?

A transformer is the device specifically designed to change the voltage level in an alternating current (AC) system. It operates on the principle of electromagnetic induction, which allows it to increase or decrease the voltage of AC power while maintaining the same frequency.

The functioning of a transformer involves two coils of wire, known as the primary and secondary windings, which are wound around a magnetic core. When an AC voltage is applied to the primary winding, it creates a magnetic field that induces a voltage in the secondary winding. By adjusting the number of turns in the primary and secondary coils, a transformer can either step up (increase) or step down (decrease) the voltage.

This ability to modify voltage levels is crucial in electrical distribution systems, where high voltages are often used for efficient long-distance power transmission, and then stepped down to lower voltages suitable for consumer use.

Other devices in the options have different roles. Capacitors and inductors are used for energy storage and filtering, but do not change voltage levels. Resistors are used for controlling current and managing power dissipation in circuits, but they do not modify voltage. Therefore, the distinct capability of a transformer to alter voltage levels in AC systems makes it the correct answer.