Answer:
D.
Step-by-step explanation:
For two organisms to breed successfully and produce viable offspring that are fertile, both must belong to the same species with the same number of chromosomes.
Postzygotic barrier often occur when two organisms of different species and chromosomes numbers mate. If successful, the hybrid they produce remain sterile. This is what happens when goat with chromosome number of 60, mates with sheep with chromosome number of 54.
Postzygotic barrier is responsible for the sterility of the hybrid they produce, as a result of difference in chromosome number between sheep and goat.