So option A (species with incomplete mating barriers is the correct answer).
What is Hybrid Zone?
A hybrid zone is formed when the ranges of two interbreeding species or diverging intraspecific lineages collide and cross-pollinate. Hybrid zones can emerge in situ as a result of the evolution of a new lineage, but they usually result through secondary contact of the parental forms following a period of geographic isolation that permitted their differentiation (or speciation). Hybrid zones are valuable in understanding the genetics of speciation because they can give natural examples of differentiation and (rarely) gene transfer between populations representing a single species and numerous species in reproductive isolation
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