Correct Answer: Phytoplankton

Solution : The correct option is Phytoplankton .

Phytoplankton are free-living organisms and are found on the upper layer of water bodies. It releases oxygen. This organism is responsible for almost half of the Earth's photosynthetic activities. They form the base of the marine food web, playing a crucial role in the Earth's ecosystems, and the global carbon cycle. Phytoplankton include various types of algae, such as diatoms, dinoflagellates, and cyanobacteria.