Answer:
a. the mongol empire
Step-by-step explanation:
Mongols lead by the Great Genghis Khan, established the largest land empire of all the times.
The Mongol Empire stretched around 30 km2 from Asia into Central Asia Europe.
Groups of barbaric warrios coming from the Mongolian plateaus, came into central Asia and China, forming a huge land mass under the will of Genghis Khan.
They were fierce in battle and the powerful cavalry played a key role , since horse men and warrios were tactically superior
After Mongols joined politically recently after finally designating a supreme leader who also happened to be a noble ruler.
They had conquered the lands shown in the image below
(max territorial extent around 1279) :