The Mongols created the largest land empire in history. How was it possible for such a small group to accomplish this? Asian countries were in an extremely divided state. They had the greatest military skill and weapons of their time. They were pastoral nomads who knew the land well. They incorporated those they conquered into their culture.
"They had the greatest military skill and weapons of their time" would be the best option from the list, since the Mongol fighters were known across the continent as being the most skilled fighters in the region.
