Answer : The correct answer is-
This means that they cannot produce viable offspring and are reproductively isolated.
Mule (which is a sterile that is infertile animal) is produced by mating of horse and donkey.
According to Biological species concept of Ernst Mayor, a group of organism belong to same species if they produce viable and fertile offspring.
This means that organisms belonging to different species are reproductively isolated that is unable to produce viable and fertile offspring.
Thus, reproductively isolated is the right answer.