A single group of fish is called a:

School

The answer is school. A group of fish is called a school. The word "school" comes from the Dutch word "schole", which means "troop" or "crowd". Fish schools are often very large, and can contain hundreds or even thousands of fish. Schools of fish swim together for a variety of reasons, including protection from predators, finding food, and migrating.

The other options are not correct. A community of fish is a group of different species of fish that live together in the same area. A bunch of fish is a small group of fish, and a group of fish is a general term that can refer to any group of fish, regardless of size or composition.