Question

horses and donkeys can mate, but they produce a mule, which is always a sterile animal. this means that they cannot produce viable offspring and are isolated.

Explanation:

In biology (a subfield of Natural Science), reproductive isolation mechanisms include post - zygotic isolation. When two species (horses and donkeys) can mate but their offspring (mule) is sterile, this is an example of post - zygotic reproductive isolation, specifically hybrid sterility. So the term to fill in the blank is "reproductively" (more precisely, in the context of reproductive isolation, the key concept here is reproductive isolation, and the type is post - zygotic due to hybrid sterility).

Answer:

reproductively