speed is the length (magnitude) of velocity
if speed changes velocity changes.
velocity has a direction, whereas speed is just a length, so velocity can change direction without changing length. This counts as a change in velocity.
acceleration is the change in the length and/or direction of velocity, so if there's acceleration, velocity is varying.
displacement is distance from a fixed point (usually the origin) so displacement technically could have any value whatsoever, regardless of distance traveled: imagine a particle that starts its trip 1 mile from the origin. But for the purposes of this problem, I suppose they're talking about a particle that starts its trip at the origin, in which case displacement can't be greater than distance traveled...