Answer: A. If populations can interbreed, they are considered one species.
Reproductive isolation is required for speciation to occur as it involves all of the mechanisms involved in evolution, including the behaviors and the physiological processes necessary for speciation. In speciation, new species emerge through evolutionary processes, and without reproductive isolation, the members of the species will interbreed freely. With reproductive isolation, however, members of different species are prevented from producing offspring or the offspring are rendered sterile. All of these barriers help maintain the integrity a species has, by reducing the flow of genes between interrelated species. If populations can interbreed, they are considered one species. With this, the integrity of species is maintained and complications are prevented.