Answer:
a. magnetic field
Step-by-step explanation:
A current carrying coil produces magnetic field and a changing magnetic field induces emf. In a transformer, when current flows in the primary coil, the magnetic field in produced which induces emf in the near by secondary coil. This results in current to flow in the secondary coil. There are two types of transformers- step up transformer has higher number turns in secondary coil and thus it steps up amount of induced emf. In a step down transformer, the number of turns in the secondary coil is less. So the induced emf reduces.