An ac generator is based on which phenomenon?

The correct answer is Option (2) → Electromagnetic induction

In an ac generator a coil is rotated in a magnetic field. The effective area of the coil facing the field, \( A\cos\theta \), keeps changing. So the magnetic flux through the coil changes, and an alternating emf is induced in it by electromagnetic induction.

NCERT (2026-27):
AC Generator – “The phenomenon of electromagnetic induction has been technologically exploited in many ways. An exceptionally important application is the generation of alternating currents (ac).”