Which term best defines a population or group of populations that are all interbreeding?

The term that best defines a population or group of populations that are all interbreeding is biospecies. This term is often used to refer to a biological species concept where a species is defined by its ability to breed and produce viable offspring in natural conditions. In this context, "bio" reflects the living organisms, while "species" denotes a group capable of interbreeding.

Using the biospecies concept emphasizes the importance of reproductive isolation, which helps maintain the genetic integrity of the populations. Within a biospecies, individuals share a common gene pool, facilitating gene flow among interbreeding members. Thus, this term effectively captures the essence of the concept of a population that can interbreed and produce fertile offspring, distinguishing it from other groups that might isolate based on different criteria.

While the other terms such as genospecies, genetic unit, and reproductive community may reference genetic relationships or reproductive behaviors, they do not specifically highlight the reproductive isolation and interbreeding characteristic that is central to the definition of a biospecies. Therefore, biospecies is the most suitable term for defining such a population or group of populations.