The Western Roman empire did not experience devastating climate change. The other answers did contribute to the fall of Rome, indicating the complexity of its decline. It is important to remember that this decline was an extremely slow and drawn out process. Just as there is truth to the saying "Rome was not built in a day", so too did it not fall in a day.

The Battle of the Catalaunian Fields was fought in the middle of the fifth century, in the midst of the decline of the Western Roman Empire. It was fought between an alliance of Roman and Visigothic soldiers against the forces of Attila the Hun. The battle ended in a stalemate, but can be best understood as part of the decline of Rome. The Roman army was unable to match the range and tactics of Attila, and massive parts of Roman Europe were pillaged.

The correct answer is relationships with neighboring countries because the new government tried to model themselves after Constantinople. If you chose separating from the church, remember that Constantine brought Catholicism into Rome. They were not trying to separate until later in the Roman Empire. If you chose colonizing Asia or Africa, remember that these conquests did not begin until way later as well. Keep time frame in mind while making your choices.  

Odoacer led a revolt of the Roman armed forces during the waning days of the Roman Empire. He deposed the last Roman Emperor, Romulus Augustus, in 476 CE, and brought to an end the rule of the Western Roman Empire. Odoacer took the title of First King of Italy and enjoyed the political support of the Roman Senate.

The Roman Empire was divided into western and eastern empires during the reign of Diocletian in 285 CE. The Western Roman Empire would last for almost two centuries, before succumbing to a series of barbarian conquests and military coups; the Eastern Roman Empire, however, would last until the fifteenth century. The Byzantine Empire is the name generally used by historians to refer to the Eastern Roman Empire after the fall of the Western Roman Empire.

Romulus Augustus was the last emperor of the Western Roman Empire. He reigned briefly in 475 and 476 CE, before being deposed by Odoacer. The end of Romulus Augustus’ reign is said to mark the end of the Western Roman Empire, although the transition was hardly smooth.

Alaric the Visigoth was the leader of a Germanic group of people (called the Visigoths) in the late fourth and early fifth centuries. He led a devastating sack of the city of Rome in 410 CE, which contributed heavily to the decline of the Western Roman Empire. It was also of great symbolic significance for people living in this time period, because it was the first time Rome had been sacked in more than seven hundred years.