A. A community is a group of populations of different species in a given area.
B. A population is a group of organisms of the same species living in a given area.
C. An ecosystem is a community of organisms that interact with each other and their surroundings or environment. An ecosystem has two major components, biotic components (the living part) and abiotic components (the non-living part).
D. The biome is a complex of different type of communities which are maintained under more or less similar climatic conditions.
So, the correct answer is 'An ecosystem'.