The correct answer is - B. Pope Leo I asked them to, and they were weakened by disease and poor food supply.
It is debatable about the reasons for the Hun's retreat from the northern part Italy, but the most commonly accepted theory is that Pope Leo I took things into his own matters and went on to meet Attila in order to convince him to make peace, not war with the Romans, and somehow he managed to do it. Also, very common theory is that the Huns suffered from famine, and that it was another big reason why the Huns returned to the plains of Eastern Europe.
