Theodosius was the first instill Christianity as the official religion after Constantine I had introduced it to the people of the Roman Empire. This question is all about time frame. If we remember that Constantine brought religion to Rome, then we also know that Theodosius I was the first ruler after to make it official. It would be tricky to confuse which Theodosius claimed the official religion. The other two options were no where near Constantine's rule of Rome. 

An ecumenical council is an assembly of Christian theologians who convene to debate and establish official church doctrine. The first ecumenical council was the Council of Nicaea in 325 CE. There were many other ecumenical councils throughout the remaining history of the Roman Empire, such as the Council of Chalcedon.

The Council of Nicaea was convened in 325 CE to debate issues surrounding Christianity. By this time, the nascent religion of Christianity had spread throughout the Roman world. Emperor Constantine converted to Christianity and, in doing so, made it inevitable that the religion would soon become the official religion of the Roman Empire. The Council of Nicaea was convened to debate theological issues, such as the nature of Christ’s divinity.

The Council of Nicaea was convened on the orders of Constantine I in 325 CE. The purpose of the Council of Nicaea was to debate various theological issues, such as the true nature of Christ’s divinity.

The Nicene Creed was established by the Council of Nicaea in 325 CE. The Council of Nicaea was the first ecunemnical council in the history of Christianity (there have since been countless). It was convened to debate issues of Christ’s divinity and to ensure universal agreement among the divergent theological schools of thought. The Nicene Creed is a profession of faith that states that Christ is the Son of God, that he descended from heaven to save us, and that he died to absolve our sins.

The Battle of Milvian Bridge was fought in 312 CE, between rival forces in the Roman Empire. According to primary sources, Constantine had a dream in which God visited him and instructed him to fly the Christian cross as his standard. Constantine, who won the battle, felt like this was divine intervention and enthusiastically embraced Christianity throughout the remainder of his life.

The Edict of Milan was issued in 313 CE by the Emperor Constantine (and the Western Roman Emperor Licinius). It formally ended the policy of religious persecution of Christians in the Roman Empire. It also established the official state tolerance of Christian worship, leading to Christianity later becoming the official religion of the Roman Empire during the reign of Theodosius I.

The persecution of Christians in the Roman Empire arguably began with the execution of Christ and certainly was commonplace throughout the first and second centuries. The height of Roman persecution of Christians, however, was not reached until early in the fourth century, during the reign of Diocletian. The so-called Diocletianic Persecution began in 303 CE, with an edict issued by the rulers of the Tetrarchy stating that all Christians had to conform to Roman worship on penalty of death. Many Christians were executed in brutal circumstances during this time, yet it was just a decade later that Constantine issued his Edict of Milan providing for religious tolerance of Christians in the Roman Empire.

The capital city of the Roman Empire was officially moved from Rome to the ancient Greek city of Byzantium in 330 CE. 

The city was renamed to Constantinople in honor of emperor Constantine I (the Great).

The correct answer is "tetrarchy." A tetrarchy is a form of government that is split between four different heads of government. The key here is the concept of decentralizing power. The other forms of government put the power in one person's hands, unlike a tetrarchy. This move by Diocletian ended the crisis of the third century.