Answer;
A) island forms and mainland forms descended from common ancestors.
Explanation;
-Islands tend to host very distinct but limited sets or species; on a given island a spectacular collection of one related group of species would be present and thriving, but many entire families or even higher taxa would be completely absent.
-All the species on the islands and the nearest continent are closely related to one another, however, each species had a set of adaptations that suits it for a particular environment. This means that the island forms and mainland forms descended from common ancestors.