The correct answer is a. phytoplankton.
Phytoplankton or microalgae are unicellular photosynthetic organisms. Dinoflagellates and diatoms are the two classes of phytoplankton. They float on the surface of the ocean water and are primary producer in the ocean. They provide food for marine animals such as whales, shrimp, snails and jellyfish. In presence of many nutrients in the ocean, phytoplankton may grow out of control to form harmful algal blooms.