(Fish : Shoal) , (Cow : Herd) , (Sheep : Flock) , (Man : Mob) , (Bee : Swarm)
Option:
Justification: In all other pairs, second is a collective group of the first.

(Fish : Shoal) , (Cow : Herd) , (Sheep : Flock) , (Man : Mob) , (Bee : Swarm)
Justification: In all other pairs, second is a collective group of the first.