Which group of organisms is primarily composed of drifting plants?

Phytoplankton are a diverse group of microscopic, photosynthetic organisms that float in aquatic environments, primarily in the sunlit upper layers of oceans, seas, and freshwater bodies. They are considered drifting plants because they derive energy from sunlight through photosynthesis, similar to terrestrial plants, and they spend most of their life cycle suspended in the water column. Phytoplankton serve as a foundational component of the aquatic food web, providing essential nutrients and energy for various marine and fresh-water species, including zooplankton, fish, and larger marine animals.

Understanding the roles of other groups can clarify why phytoplankton is the correct answer. Nekton refers to actively swimming organisms, like fish and squid, that navigate through the water rather than drift. Piscivores are organisms that primarily eat fish and are more associated with larger, predatory species. Zooplankton are small, often microscopic animals that primarily feed on phytoplankton rather than being composed of drifting plant life.