1 and 4 are the best choices since The barbarian attacks killed many romans and the Huns sacked hundreds of cities in the Roman Empire and caused chaos. I also believe 4 is another reason for the decline, since the religion started about 400 years before the fall of the empire and sparked many different believes and divided the Empire through religion.
C. Citizens and their government agree to enter a social contract.
People moved West because the government offered new settlers a lot of land for farming at a very low cost
