Okay, Josh1548, we could mention that there're many aspects that we cannot appreciate in an aquarium model of saltwater ocean, like the richness of species, their numerous biotic and abiotic ecological interactions, and the effects of the different environmental variables, like salinity, pressure, and temperature.
Having this clear, a saltwater aquarium would not represent some important values or characteristics present in the ocean, like the limited range of depths, that gives the possibility to a higher number of species use a higher number of biological niches, at differents pressure, temperature and salinity degrees.
So the correct answer is C. It can show only a small range of ocean depths.
The answer shouldn't be A, B, or D, since if it's about an ocean model, freshwater will be restringed to other ecosystems, like rivers, or lakes, plants are not present in the ocean, only other species belonging to algae classification, and mangrove only at intermediate salt concentrations, at areas where freshwater mix with salt water.