Maximum CO2 fixation occurs in
About 90% of oxygen releases from algae of oceans (or aquatic), terrestrial plants contribute only remaining %. Comes to carbon reduction /fixation, it would be phytoplanktons since earth covers more water than land. Comes to per plant basis definitely it is terrestrial trees preceeds terrestrial plant then aquatic plants, in other words, it has more carbon reduction area /leaf area than aquatic.
So, the correct option is 'Phytoplankton'.