Assertion (A): Transformer is useful for stepping up or stepping down voltages. Reason (R): Transformer is a device used in D.C. circuits. In the context of the above two statements, which one of the following is correct?

The correct option is C because of the following reasons: Assertion A is true: The fundamental function of a transformer is to change the voltage levels of alternating current. It can either increase the voltage, known as a step up transformer, or decrease it, known as a step down transformer. Reason R is false: Transformers operate on the principle of electromagnetic induction, which requires a constantly changing magnetic field. This is only provided by Alternating Current. In a Direct Current circuit, the magnetic field is constant, so a transformer cannot function and may even be damaged due to overheating. Since the assertion is a factually correct statement about the utility of a transformer and the reason is a factually incorrect statement about its application, option C is the correct choice.