The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Constantine finally seized control of the entire Roman Empire after this major event: the Battle of Milvian Bridge in 312.
The get total control of the Roman Empire, Constantine had to defeat Maxentius in that final confrontation. Before the battle, to be precise, one night before the battle, Constantine had a dream in which an angel appeared and told him if he accepted the Christian religion, he would win the battle. As this happened, after the battle, Constantine accepted and tolerated the Christians in the Empire.
