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.

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem