Which term refers to a group of organisms that are so similar they can breed to produce fertile offspring?

The correct term for a group of organisms that are so similar they can breed to produce fertile offspring is "species." A species is defined by its ability to reproduce and produce viable offspring that can also reproduce. This biological concept emphasizes the reproductive isolation of one group from others, which is a key criterion for classifying organisms into different species.

In contrast, the term "community" refers to a collection of different species that coexist in a particular environment, while "ecosystem" encompasses the living organisms and their physical environment interacting together as a system. "Population," on the other hand, refers to a group of individuals of the same species living in a specific area. Hence, the definition and characteristics of a species specifically focus on reproductive compatibility, making it the most accurate choice for the question posed.