Choose the correct collective noun from the options below and fill in the blanks.
A ………… of fish.

School

The correct collective noun for a group of fish is school or shoal. So the answer is A school of fish.

The other options are incorrect. Swarm is a group of insects or other small animals, not a group of fish. Crew is a group of people who work together on a ship or other vehicle, not a group of fish. Pack is a group of wolves, not a group of fish.