Hint: Study of the organism and how they interact with the environment and with other species present in the same environment refer to ecology and the basic structural and functional unit of the ecology is an ecosystem.
Ecological hierarchy refers to the arrangement of biological organisms concerning one another. At the basic level, it comprises individual organisms, and at this level, any interaction with other organisms is not considered. As the level of hierarchy goes up there is an establishment of several complex relationships between different species.
In an ecosystem, there are five ecological hierarchy levels;
-Organism: The lowest level of ecological hierarchy including both unicellular as well as multicellular organisms. All living species in this hierarchy level exhibit all characteristics required for the existence of life.
-Population: It is a group of individuals of a single species living together within a specific geographic area and competes with each other for resources.
-Community: Several populations of different species interact with each other which form communities. This level mainly focuses on the relationships between the various organisms of different species.
-Ecosystem: Ecosystem is an interaction between all living species and non-living things present in the environment. A community is a part of the ecosystem but it is not the entire ecosystem.
-Biosphere: Biosphere is the highest level of the ecological hierarchy. It is the global ecological system that contains all living organisms and other factors that support life. It mainly refers to the part of the earth’s crust.
-Biome: It consists of a specific area that consists of many ecosystems but it is a part of the biosphere.
Hence, the correct answer is option (D).
Note: The basic unit of ecological hierarchy or ecological levels of the organization is an individual organism. The biome is an ecological concept that is related to an ecosystem. Moving up the ecological hierarchy the complexity increases to describe the relationship between organisms.