C. <span>mandating religious and cultural tolerance throughout his empire
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<span>The Russian serf system arose as peasants fled to the protection of large landowners to avoid Mongol taxes.</span>
I'm not sure if the emphasis is to be put on the fact that the <em>Pope</em> declared or that the event occurred. I'll answer both.
It was significant that the Pope declared it because he was pretty much the only person who had the ability to declare such a thing. Even though the kingdoms of Europe had different rulers, most kingdoms were based on the Catholic religion. Therefore, the Pope had influence. If he declared something, it was as good as God declaring it.
It was significant that a Holy Roman Emperor was declared because it was the first step -- of many -- to achieving German unification. Before Charlemagne, Germany was nothing more than the Germanic tribes. The Holy Roman Empire, which lasted until 1806, kept these kingdoms united.
Details please, what are they
The United States kept to itself because it was following an isolationist policy at the time
