The driving power from the prime-mover driving the alternator is lost but the alternator remains connected to the supply network; the field supply also remains on. The alternator will
1
get burnt
2
behave as an induction motor but will rotate in the opposite direction
3
behave as a synchronous motor and will rotate in the same direction
4
behave as a synchronous motor but will rotate in a reverse direction to that corresponding to generator action