A transformer is used:
(A) To transform electric energy into mechanical energy
(B) To obtain suitable DC voltage
(C) To transform AC into DC
(D) To obtain suitable AC voltage

Hint: The word “transformer” gives a hint of transforming some quantity, this is confusing. Instead, transformers are used to change the voltage. Working of a transformer is based on the concept of Lenz’s Law.

Complete step by step answer:
Transformer: It is an electrical device which is used to drop or increase the voltage. It consists of primary coil, secondary coil, load impedance and AC signal.
Number of turns in primary coil is denoted by ${N_P}$
Number of turns in secondary coil is denoted by ${N_s}$
If ${N_P} > {N_s}$ , then it will work as a step down transformer. The voltage of AC signals in the secondary circuit will decrease in this case.
If ${N_P} < {N_s}$ , then it will work as a step-up transformer. The voltage of AC signals in the secondary circuit will increase in this case.
Current flowing in primary coil changes, this induces magnetic flux which gets transferred to the secondary coil. Change in magnetic flux will further lead to electromotive force or voltage. This will lead to flow of current but in the opposite direction to the primary coil. This is because of Lenz’s law.

Note: The word “transformer” is a misnomer. It is not used to transform anything. But instead, a transformer is used to step up or step down the voltage. Make sure to remember this exception. Since DC signals are given at input of the transformer then there will be no change in current. Constant current cannot induce new currents hence a DC transformer is not a straightforward possibility. Also, DC signals cannot be obtained from AC signals using this circuit. Hence option B and C are not possible.
Electric motor is a device which is used to convert electrical energy into mechanical energy. Therefore, option A cannot be a function of the transformer.
Therefore, we are left with only one option I.e., D. Thus, the correct solution is option D.