The correct answer to this question is C, Zero. It must be zero for an balanced undirectional, steady torque to occure in both fields. Speed wrt must be calculated first. Then, supply frequency can. Slip frequency, which is abbreviated sf, can be used to also calculate speed wrt.
To solve this question, the formula nr + sns= ns (l-s) + sns= ns r.p.s. With respect to all other rotor speeds, the stator and rotor fields are stationary to each other. The induction motors are unable to run at a synchronous speed because their would be no directional torque, furthermore proving the answer to be zero.