In which case is there an induced electromotive force?

An electromagnet operating on direct current is held inside a second coil of wire.

An electromagnet operating on alternating current is held inside a second coil of wire.

A permanent bar magnet is held inside a coil of wire.

Field lines from earth's magnetic field cross the area of a coil of wire.

Question

In which case is there an induced electromotive force?

   An electromagnet operating on direct current is held inside a second coil of wire.

       An electromagnet operating on alternating current is held inside a second coil of wire.

       A permanent bar magnet is held inside a coil of wire.

       Field lines from earth's magnetic field cross the area of a coil of wire.

anonymous · Answer

EMF proportional to change of magnetic flux per time through a loop, so rule out A, C, D.