Two different species of frogs mate successfully and produce fertile hybrid offspring. however, the offspring of the hybrids do not develop completely to adulthood. this is an example of which type of reproductive barrier?

There are three types of hybrid reproductive barriers. The one described is called hybrid breakdown. In hybrid breakdown as the hybrid organisms continue to reproduce each sequential generation becomes less viable, healthy and loses it's ability to reproduce. The other two hybrid reproductive barriers are hybrid sterility and hybrid inviability. Hybrid sterility results in healthy organisms that can not reproduce at all since thy are sterile. Hybrid inviability results in organisms that are not strong enough to survive and often die while juveniles.