What are zooplankton primarily composed of?

Zooplankton are primarily composed of tiny single-celled and multicellular animals. This is correct because zooplankton encompasses a diverse group of aquatic organisms, which include both single-celled protozoans and various forms of multicellular organisms, such as tiny crustaceans like copepods and larval stages of larger animals. This composition allows them to play a significant role in aquatic food webs, as they serve as a crucial food source for larger aquatic animals, including fish.

The other options do not accurately represent the composition of zooplankton. Restricting them to only single-celled organisms or only multicellular organisms overlooks the diversity within this group. Additionally, limiting zooplankton to only plant-based microorganisms is incorrect, as zooplankton are primarily animal-based, contrasting with phytoplankton, which refers to plant-like microorganisms.