The best definition of an ecosystem is c) the living organisms and their environment interacting in a certain area.
An ecosystem includes all the living organisms (plants, animals, and microorganisms) in a specific area, as well as the non-living components (such as air, water, and soil) that they interact with. These interactions form a complex web of relationships that sustain life and maintain the balance of the ecosystem.
Option a) refers only to the abiotic factors, which are the non-living components of the environment. Option b) describes a population, which is a group of organisms of the same species living in a certain area. Option d) is too vague and does not capture the dynamic interactions that define an ecosystem.