The circuits where transformers are used:

The correct answer is Option (1) → ac circuits

A transformer works on mutual induction: an emf is induced in the secondary coil only while the magnetic flux linked with it changes.

An alternating current in the primary produces a flux that changes all the time, so an alternating emf appears in the secondary and the voltage can be stepped up or down. A steady direct current produces a constant flux, which induces no emf in the secondary (apart from a brief pulse at switching on or off). So transformers are used only in ac circuits.

NCERT (2026-27):
Transformers – “… This is done with a device called transformer using the principle of mutual induction. … When an alternating voltage is applied to the primary, the resulting current produces an alternating magnetic flux which links the secondary and induces an emf in it.”