Which of the following unify the provinces under the roman empire

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

it should definitely consists of the total amount of the entry which hindered the diary of how most conflictions and opinions in the 1800s experiences.