The Battle of Chrysopolis was fought in 324 CE between the forces of Constantine and the forces of Licinius. Licinius and Constantine were both rulers of the Tetrarchy and, as the efficacy of the Tetrarchy dissolved, came to be political rivals. Victory at Chrysopolis allowed Constantine to consolidate his power as the sole Emperor of Rome and ended the Civil Wars of the Tetrarchy.

The Battle of Milvian Bridge was part of the Civil War of the Tetrarchy. It was fought in 312 CE, between the forces of Constantine and the forces of Maxentius. It ended with the death of Maxentius and victory for Constantine’s forces. It was instrumental in the rise to power of Constantine.

Atilla the Hun was the leader of the Hunnic Empire during the middle of the fifth century. The Huns were a nomadic people who led a series of devastating attacks on the barbarian populations of eastern and central Europe. These attacks led to a massive displacement of the Germanic people who were forced to migrate further into the heart of the Roman Empire to escape Atilla’s catastrophic raids. This contributed to the weakening of the Roman Empire, particularly in the west. Attila invaded Italy in 451 CE, but was stopped before he could sack the city of Rome.

The First Council of Nicea (325 AD) created the Nicene creed as a way to unify the fragmenting and diversifying beliefs in Christianity developing throughout the Byzantine empire. It establishes and explains the relationships between God, Jesus Christ, and the Holy Ghost. The other answers are incorrect because it was not crafted as legislation for political purposes, and was written to counter Arian Christianity, which questioned elements of the orthodox faith.

The First Council of Nicaea was convened by Emperor Constantine I in 325 CE. The main purpose of this council was to debate the nature of Jesus Christ in relation to his divinity according to Scripture. 

The council concluded that Jesus Christ was divine, co-eternal with God the Father as according to Christian doctrine.

Emperor Constantine I (also called Constantine the Great) supposedly had a vision during the Battle of Milvan Bridge in 312 CE in which he saw a cross of light hovering above the battlefield along with the words "in this sign, conquer".

The following year, the Edict of Milan granted tolerance to all religions, including Christianity. Christianity would later go on to become the state religion of the Roman Empire in 380 CE.

The Roman Empire had a long and sometimes contentious relationship with Judaism, however the Edict of Milan did not mention them, so Judaism would not be a good choice. Islam only became a popular and widely practiced religion after the fall of Rome, so that too would not be a good choice. Buddhism and Hinduism are ancient world religions, however the Roman Empire did not have any notable exposure to them, so they would not be good choices. Finally, the Edict of Milan specifically called for tolerance of Christianity, so that would be the best choice.

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.