The correct answer is - A. The Mongols unified regions along the Silk Road, allowing for safe transport of goods.
The Mongols, though often portrayed as brutal and barbarians, were actually the ones that did the opposite of what was expected from them. After conquering the territories through which the Silk Road was going, they put in a big effort to make it as secure as possible, to a degree where some European traders wrote that you can go on foot from one end to another on the Silk Road and you will not be robbed or should fear for the safety. The Mongols managed to reestablish the Silk Road and even improve, thus making the trade between Europe and the Far East flourish until they were in charge.