For what purpose are transformers used? Can they be used with a direct current source?
A. To step up the a.c. voltage ; No, a transformer cannot be used with a d.c. source.
B. To step down the a.c. voltage ; No, a transformer cannot be used with a d.c. source.
C. both A & B
D. None of the above

Hint: Transformer is a device that transforms the low voltage to high voltage or vice versa. It is based on the principle of mutual induction.

Complete step by step answer:
There are two types of transformer step up transformer or step-down transformer. In the step up transformer it increases the alternating current voltage. In step-down transformers it decreases the alternating current voltage. It increases or decreases the voltage level depending on the relative number of turns between the primary and secondary side of the transformer. If more turns in the primary coil than the secondary coil then the voltage will decrease so it is called a step down transformer. If there are fewer turns on the primary coil than the secondary coil then the voltage will increase so it is called a step up transformer. So, the main purpose of the transformer is to step up the ac voltage or step down the ac voltage.
Transformers cannot be used in direct current supply. The reason is that it is very simple because the transformer works on the principle of mutual induction, for which current in one coil must change uniformly as induction can only take place due to changing flux through the coil, which can only be changed in case of fix geometry of transformer by changing magnetic field through it which is obtained basically by obtaining change in current(AC current). If dc supply is given, the current will not change due to constant supply and the transformer will not work.
Ans it is also very dangerous if dc is supply in transformer the winding will draw a very high current which generate the excessive heat and may cause permanent damage to the transformer
$\therefore$ The correct answer is option C, both A and B are correct

Note: In our daily life transformers are used such as rectifiers, voltage regulators, power supplies, voltage suppliers, ammeter, voltmeter etc.