The correct answer is: political instability, military conflict, and economic crisis.
Political instability: at the end of the 4th century AD, the Roman empire was going through a political crisis. While the emperor Theodosius tried to handle the social uprisings between Christians and non-Christians, he was struggling against the usurper Magnus Maximus and the empire was facing cases of corruption in the political sphere that diverted public funds from the military needs. Due to these reasons and for administrative purposes, Theodosius decided to establish Christianity as the official religion of the empire and divided the empire into two parts: the Western Roman Empire, with its capital in Ravenna, and the Eastern Roman Empire, with its capital in Constantinople.
Military conflict: while the Roman Empire was facing these internal problems that weakened it, there were urgent problems in its borders since many barbarian invaders were attacking Roman positions from the outside. The Huns from the East, led by Attila, devastated a great portion of the empire, Saxons invaded Britain, Goths and Lombard people from the North as well invaded Italy and Hispania. The Roman army for the first time was not in the position of facing so many fronts at the same time.
Economic crisis: the enormous Roman administrative device was going through a financial crisis. It was so big that it did not find the necessary resources to satisfy its needs. Gold mines that used to fuel the economy were very far away, difficult to achieve, and the Empire had to make a large number of official coins out of copper provoking a great devaluation of the Roman currency.
