Community: deals with groups of organisms living or somehow interacting with one another in the same field/habitat.
Population: deals with same species of animals in a group in a place
Ecosystem: deals with all living organism and where they are at in an environment.
Biosphere: deals with the area in which there is life on this planet and integrates all living species and their relationship with one another and their interactions with other parts of the planet such as the atmosphere/ecosystems.
Hopefully this helped and good luck!! If you need any clarifications just let me know.
Biome, though some might say ecosystem instead. Biomes are very large ecosystems that cover broad areas.<span>If they specify "one" reef, then they are probably looking for "ecosystem"</span>
