Two populations of frog species are separated by a group of mountains. A tunnel is built through the mountains, through which some of the frogs are able to safely navigate, and attempt mating with frogs on the other side of the tunnel. However, their heroic feat is for nothing, because their offspring all produce sterile young. This scenario is an example of

Post reproductive isolation → Hybrid sterility

Explanation:

The biological concept of species states that individuals of a species can not mate and reproduce with individuals of another species. But if they get to reproduce, the progeny will not be viable or fertile. There will not be any reproductive success.

There are different reproductive isolation mechanisms, which are barriers that inhibit or interrupt the genetic flow between different species.

Reproductive barriers are isolation mechanisms that prevent mating between two or more species. The prezygotic mechanism avoids fertilization between individuals of different species, while the postzygotic mechanism impedes the zygote to develop and reach the adult stage.

Postzygotic mechanisms or barriers include

In the exposed example, it seems that the mountains separating the frogs´ populations made a place for the development of postzygotic barriers, specifically hybrid sterility. Frogs from one population get to mate and produce offspring with the frogs of the other population, but their progeny is sterile.  

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.