In the world of bartering, you can only obtain a goods by exchanging it with other goods. Which mean that your option for occupation would be limited to a producer for a certain goods.
In a world with money, you don't have to do that. You can choose occupation that is not related to production (such as painters, singers, or Teachers) and in exchange for your work, you can obtain money to buy the goods that you need.
The answer that best applies to both the Tang and Song dynasties would be C. A Reliable food production and distribution system. This gave both dynasties the ability to thrive.
