Which one of the following is a decomposer?

fungi,algae,protozoa,insect

The decomposer are organisms that feed on the dead organisms. They get energy from the nutrients present in the dead organisms. They are one of the important organisms of the ecosystem because without decomposer, the dead organisms cannot be decomposed.

Common decomposer are fungi and bacteria.

Protozoan and insect are heterotrophs which may acts as decomposers some times.

But algae comes under plant kingdom which is an autotrophs.

Thus, fungi is a pure decomposer. Example: mushroom.