An ecosystem is accurately described as `a community of organisms together with the environment in which they live` which is option 3.

Option 1 is only partially correct as it involves `a community of organisms interacting with one another` however, it lacks the component of interaction with the environment which is a key element in an ecosystem.

Option 2 which is `that part of the Earth which is inhabited by living organisms` is too broad for an ecosystem. This describes biosphere rather than an ecosystem.

Option 4, `the flora and fauna of a geographical area` primarily describes the biodiversity of a particular region, but does not account for the interactions and interdependency that exist between the organisms and their environment which are central to the definition of an ecosystem.