Answer: The correct answer is- migration.
The term migration can be described as the movement of organism/ population from one region to another region.
As per the information in the question, raccoons family living in the center of a forest leaves and joins a population of raccoons living near the edge of the forest.
Due to migration of new population, overall size of edge population is increased and because of interbreeding between raccoons, a certain trait ( that could be earlier present only in the edge population) in the raccoon population becomes more common over a period of years as now it would be present in both the former edge population as well as the migrant edge population.
Thus, the factor caused the migrant raccoon population to evolve is migration.