Final answer:
The ecosystem is the level of biosphere hierarchy where different species interact with each other and with abiotic factors. It includes biotic components like animals and plants as well as abiotic components such as weather and soil.
Step-by-step explanation:
The ecosystem is the lowest level of the biosphere hierarchy to include interactions between different species. An ecosystem consists of all the living things (biotic factors) in a given area interacting with all of the nonliving parts of the environment (abiotic factors) like weather patterns and soil conditions.
This includes various animal species, plant species, as well as the myriad interactions among them and with their non-living surroundings. Thus, the correct answer to the student's question is c. interactions between different species.