Cogging is the tendency of the rotor to 'lock' with the stator slots.

Magnetic locking occurs due to minimum reluctance when teeth align.

Designers often select $S_r$ such that $S_r$ is not a multiple of $S_s$.

Skewing of rotor slots is also used to minimize cogging and noise.

Smooth motor acceleration

Reduced noise and vibration

Elimination of dead-start points

Complex manufacturing for non-standard slot counts

Increased leakage reactance if slot combinations are poorly chosen

Squirrel cage induction motors

High-efficiency industrial drive motors

Standard practice suggests that $S_r$ should not be $S_s \pm P$ or $S_s \pm 2P$ to avoid synchronous crawling.

Option B is incorrect because having a different number of slots is a design requirement, not a restriction against being lesser than.