Question

Which of the following helped unify the provinces under the roman empire

Answer 1

I believe the answer is: the law of nations

The law nations were the first known legal treaties for international laws which created by the Roman to influence various regions to adopt their law system. By imposing one same law, The roman manage to unify several provinces under one central government and increased its power

Answer 2

Here are some of the ways in which the provinces were unified under the Roman Empire:
Emperors encouraged the conquered people to build cities and made these people Roman citizens, they involved them in the government of Rome, the ideas of the Romans spread to surrounding areas.