Development of similar habits in pouched and placental mammals found in similar habitats is called
Rodents like the flying squirrel are all the more firmly identified with primates or some other placental mammal than they are to the sugar lightweight flyer, which is a marsupial. These creatures are a case of convergent evolution. Predecessors of present-day marsupials presumably split from those of current placental warm-blooded animals in the mid-Jurassic timeframe, back when Stegosaurus Allosaurus still meandered the Earth.
So, the correct option is 'Convergent evolution'.