Answer:
a) hybrid inviability
Explanation:
Two species of frogs belong to the same genus and interbreed sometimes. It means that the prezygotic reproductive barriers are not working here since the frogs of two species are able to interbreed and produce a zygote. However, the resultant offspring are less viable and do not hatch. It represents hybrid inviability wherein the hybrids of two species do not develop properly during embryonic development and die soon. It is a type of postzygotic reproductive barrier.