Correct Option: A

Induction motor is similar to a transformer with secondary short-circuited if supply is supplied across primary or primary short-circuited if supply is applied across secondary. If supply is applied across rotor, then flux which is revolving in nature is developed. It sweeps past the startor surface and cuts conductor of startor and so emf is induced in the startor which is revolving at natural but with lesser speed than the rotor. So due to this relative speed, the rotor starts to rotate. This explanation is vice-versa to the explanation, when the startor is energised with supply. So frequency of startor at the above specific condition is slip frequency.