A. Clowder
B. Troop
C. Shoal
D. Skulk
Explanation:
A group of fish is called shoal or school of fish. Most fish tend to swim in groups to protect themselves from predators. The collective noun which denotes such a group depends on the species of the fish in the group, as exemplified by the terms a troop of dogfish, a swarm of eels and a glide of flying fish.