A transformer is primarily used to change which of the following?

Transformers are electrical devices specifically designed to change voltage levels of alternating current (AC). They operate on the principle of electromagnetic induction, allowing them to step up (increase) or step down (decrease) voltage while maintaining the same power level, assuming ideal conditions with no losses.

In practical terms, when alternating current flows through the primary coil of a transformer, it generates a magnetic field that induces a voltage in the secondary coil. The ratio of the number of turns in the primary coil to the number of turns in the secondary coil determines the change in voltage. For example, if the secondary coil has more turns than the primary, the output voltage is higher (step-up transformer); conversely, if it has fewer turns, the output voltage is lower (step-down transformer).

The other choices involve concepts that do not align with the fundamental function of a transformer. Resistance levels are managed through resistive components and circuits; frequency relates to the oscillation rate of AC and is not altered by transformers; and transformers do not convert current types between AC and DC as they are solely meant for AC voltage transformation. These distinctions clearly affirm that the primary role of a transformer is indeed to change voltage levels of alternating current.