Answer:
Algae
Step-by-step explanation:
Plankton is a general term for aquatic microscopic organisms that lacks the ability to move on their own, hence, they float freely along the water currents. A planktonic organism can either be plant-like known as phytoplankton or animal-like known as zooplankton.
According to the question, algae is an example of phytoplankton that is microscopic and floats freely in water. It is plant-like due to its possession of Chlorophyll. Hence, phytoplanktons are the major producers in an aquatic ecosystem. Other examples in the options are macroscopic motile organisms