Which of the following people is responsible for making the christian faith the official religion of the roman empire?
a. pope john paul i
b. emperor justinian
c. julius caesar
d. emperor constantine

It was not constantine the great, that finished the "officializing", jleezy:  

although constantine did make christianity the official religion, there was many divisions in christianity, constantine called the nicene ecumenical council in an attempt to unite the christians. 

that was unsuccessful and different christian denominations actually killed each other, so a couple hundred years later, justinian, the ruler of the eastern roman empire, had another tactic to "unite"--he make the roman bishop the head of all the churches (536 ad) and then delivered rome from heathen oppression in 538. he used 35,000 troops to extinguish all opposition to rome, and that started the dark ages of papal supremacy and extermination of "heretics.". 

The answer is a hope