Final answer:
A transformer transforms voltages from one value to another and operates only on AC because it needs a changing magnetic field to function.
Step-by-step explanation:
A transformer is a device that operates based on the principles of Faraday's law of induction and is used to transform voltages from one value to another. The correct answer to the student's question is that a transformer operates only on AC (Alternating Current). This is because transformers require a changing magnetic field to induce voltage in the secondary coil, which is not possible with DC (Direct Current) as it provides a constant voltage. Transformers are integral in various applications, including power distribution systems and electronic devices, converting high voltage to lower levels for safety and efficiency.