All of the above
A transformer uses the properties of an inductor to transform the level of voltage in a circuit. AC flowing in wires wrapped around an iron core magnetizes the core, and in turn produces a changing magnetic field on the core. This changing magnetic field generates a voltage in a neighbouring coil of wire. Depending on the number of turns on the wire in the primary versus the secondary coil, a smaller or larger voltage can be induced in the secondary coil.
B The primary coil is the one that is connected to the source.
C The secondary coil is the one in which an electric current is induced.
D A larger number of secondary coils means a larger voltage.