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 assertion A is correct: transformers are specifically designed to change voltage levels — by choosing appropriate winding (turns) ratios they can step up or step down RMS voltages between primary and secondary circuits [1]. The reason R is false: transformers operate by electromagnetic induction requiring a time-varying magnetic flux, so they work with alternating (changing) currents/voltages; a steady D.C. produces no continuous changing flux and thus cannot be transformed in steady state. Transformers do not pass steady DC and are not devices for DC circuits. Therefore A is true while R (that a transformer is used in DC circuits) is incorrect.

Sources

1. [1] https://www.sciencedirect.com/topics/engineering/control-transformer