The first stage of the circuit converts a direct current (d.c.) to an alternating current (a.c.). Explain why a transformer needs a.c. to work.

A transformer operates on the principle of electromagnetic induction. This requires a changing magnetic field to induce an electromotive force (e.m.f.) in the secondary coil.

is necessary.
Alternating current (a.c.) continuously changes its direction and magnitude, which in turn produces a continuously changing magnetic flux in the primary coil. This changing flux links with the secondary coil, inducing an a.c. voltage across it. Direct current (d.c.) produces a constant magnetic field, which does not induce a continuous e.m.f. in the secondary coil.