Final answer:
To determine a 'species' in biology, reproductive isolation is the traditional method, as species are defined as groups of interbreeding populations that are reproductively isolated from other groups.
Step-by-step explanation:
In biology, defining a species can be complex because of factors like gene transfer. However, the concept is traditionally based on reproductive isolation, as per the Biological Species Concept. This approach argues that a species is a group of interbreeding natural populations that are reproductively isolated from other such groups. Therefore, the species would be best determined by reproductive isolation, answering the original question with option b).
Hybridization occurs when members of two closely related species reproduce (option c), and this can sometimes challenge the clear definition of a species. Furthermore, genetic variation, which drives the evolution of species, primarily results from mutations and sexual reproduction (option a). Speciation occurs when there is sufficient genetic variation leading to reproductive barriers.