Which of the following best defines an ecosystem?

The definition of an ecosystem as "a community of living organisms in a specific environment" accurately captures the essential components that make up an ecosystem. An ecosystem includes not only the living organisms, such as plants, animals, and microbes, but also the non-living elements like water, soil, and air that interact with these organisms. This relationship among biotic (living) and abiotic (non-living) factors is fundamental to the functioning and sustainability of the ecosystem.

In contrast, other options like a collection of rare species or only areas with living characteristics do not encompass the entirety of what constitutes an ecosystem. While rare species may exist within an ecosystem, they do not define it, since ecosystems are defined by a diverse range of organisms and their interactions. Further, the notion of "only areas with living characteristics" ignores the crucial non-living components that are vital for supporting life. Lastly, a designated wildlife reserve is a specific type of area that may contain an ecosystem but does not represent the broader concept of ecosystems in various environments.