The correct answer is B.
Induction motors which are also called asynchronous motors work on the principle of induction which is inducing a current in the rotor by changing the magnetic field of the stator.
We can differentiate two definitions of speed in induction motors:
- Synchronous speed – The speed of the changing magnetic field.
- Asynchronous speed – The speed of the rotor (which is different from the synchronous speed).
The difference between these two is called slip. Therefore, B is the correct choice.
As for the other answers, single phasing (A) occurs when one of the motor terminals is loose or disconnected making the motor run on a single-phase. No-load speed of a motor (C) is the maximum speed of a motor when there is no load applied.