Why does geographic isolation cause speciation apex?

That is because environments change and and species adapt to their environments. If the same species gets separated and then becomes geographically isolated, they will have to adapt to their respective environment. When enough of these small adaptations occur, one of them will completely become a new species that would be incompatible with the original one when it comes to breeding.

Brand c i think that should be it

The correct answer is:   [b]:   "physical systems" .