The base of the ecological pyramid is producers
Oxygen molecules diffuse fast because the main gas in a cell in Carbon Dioxide its like if it was 99.9 percent carbon that .1 percent is oxygen and carbon dioxide being what it is. Automatically overrides the amount of oxygen. Basically it is nonpolar
They are all freshwater. So the closest would be c. an estuary since it has a free connection with the sea since the sea is not freshwater.
So C.
hope this helps!