Answer:
D - use of many horses for mobility
Step-by-step explanation:
Use of horses for mobility can be considered as the one of important factors which plays an important role in their staying undefeated and it also allows them capture half of the Asia.
Every Mongolian kid was given 3-4 horses at their early ages with whom they grew up training to become a great soldiers. One of the most greatest advantage of using the 4 horses was mobility -increased speed.They usually maintain 3-4 horses as it's helps them to travel fast, they changes their ride when one of their horses get tired. This allows them to move fast and also provided them strategic advantage over their enemies in some cases .
For instance, during the invasion of Hungary they traveled at speed of 160 km per day which was not expected by Hungarians