Afemale horse can mate with a male donkey but they will produce a mule that is sterile. this is an example of what type of isolation mechanism?

The postzygotic isolating mechanisms

Explanation:

The postzygotic isolating mechanisms refer to the mechanisms that isolate members of different species reproductively after the process of fertilization.  

One of these mechanisms is hybrid sterility wherein the hybrid produced by a cross between two organisms of different species is sterile and can not reproduce.  

In the given example, the mule is an interspecific hybrid of a female horse and male donkey but is sterile. This is an example of postzygotic isolating mechanisms that prevent gene flow between horses and donkeys.