Answer:
Alternating voltage leads to electromagnetic induction which is necessary for the transformer to work.
Step-by-step explanation:
According to Oxford dictionary; an alternating current is "an electric current that reverses its direction many times a second at regular intervals".
A transformer works on the principle of electromagnetic induction. A transformer requires an alternating current which can create a changing magnetic field leading to induced voltage in the coil.
Hence, a transformer requires alternating voltage because alternating voltage leads to electromagnetic induction which is necessary for the transformer to work.