The answer is: d)It started in the Mongol Empire and spread via trade routes to all corners of Asia and the Middle East.
The origins of the plague is started in central Asia , which at that time was a part of Mongol's territory. The mongols often trade with other empires in all corners of Asia and the middle East.
When the a certain person is infected, it does not directly shows physical symptoms,. This mean that if a trader got infected, it could act as a carrier that spread the virus to other kingdom.