A. Biotic community is a group of populations of different species in a given area.
B. The ecosystem is the community of organisms that interact with each other and their surroundings or environment.
C. A population is a group of individual organisms of same species living in a given area.
D. A biome is a complex of different type of communities which are maintained under more or less similar climatic conditions.
So, the correct answer is 'Population'.