If biological species are defined in terms of reproductive compatibility, the formation of a new species hinges on .
A) gene flow
B) reproductive isolation
C) hybrid formation
D) gene pool expansion

Option-(B): If biological species are defined in terms of reproductive compatibility, the formation of a new species hinges on reproductive isolation.                            

Explanation:

There is diversity among being and we see that millions of species roam the face of land, all this happens in the tenure of centuries. While the process of evolution or creation of new species happens when a certain specie is kept inside such an environment or a very unique environment that the species is tempted to cross with another form of organism belonging to a different specie, so, this process initiates the creation of new species or organisms having a very different set of characteristics as compared to there ancestors.