The correct option is A.
Allopatric speciation is said to occur when a particular biological population of the same specie get separated from each other to the extent that their isolation prevent them from exchanging genetic materials with each other through reproduction. This type of speciation is also called geographical speciation because, the population usually get separated geographically with one been removed or restricted to a new location. This kind of separation of specie leads to the development of divergnet adaptations.
