The correct answer is: C. Frequency
A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. The voltage and current in the output circuit are different from the voltage and current in the input circuit, but the frequency remains the same.
Option A: Current. The current in the output circuit is proportional to the current in the input circuit. The ratio of the output current to the input current is equal to the turns ratio of the transformer.
Option B: Voltage. The voltage in the output circuit is proportional to the voltage in the input circuit. The ratio of the output voltage to the input voltage is equal to the turns ratio of the transformer.
Option D: All of the above. This option is incorrect because frequency is the only quantity that does not change in a transformer.