In gears, interference takes place when
(a) The tip of a tooth of a mating gear digs into the portion between base and root circles
(b) Gears do not move smoothly in the absence of lubrication
(c) Pitch of the gear is not same
(d) gear teeth are undercut
The question was posed to me during an interview.
My enquiry is from Reverted Gear Train in portion Toothed Gearing & Gear Trains of Machine Kinematics