Final answer:
The Vikings' conversion to Christianity was the factor that most contributed to their integration into European civilization.
Step-by-step explanation:
The factor that most contributed to the Vikings becoming part of European civilization was their conversion to Christianity. By the mid-tenth century, the Scandinavian kings began to assert their control and rein in Viking raids. The adoption of Christianity, which became very common by 1000, helped end the raiding period and led to the Vikings assimilating into European society.