Answer
Correct Answer is D ( so that there will be a change in the magnetic field required for induction)
Step-by-step explanation:
Transformer is a device in which electrical energy is transferred between two or more circuits.
Transformer working is based on Faraday’s law of electromagnetic induction. Transformation action in a transformer occur due to mutual induction between two or more winding.
A transformer consists of one or more winding which are coupled magnetically and separated electrically with a magnetic core. A changing or varying current in one coil of the transformer produces a changing magnetic flux due to which an emf (electromotive force ) is induced across the second coil which is wound around the same core.
Therefore a transformer needs an AC due to which a changing magnetic field will be created and due to this changing magnetic field an emf will induced in the second coil.
If DC is used in transformer then magnetic induction will not takes place since DC current will remain constant