The correct answer is C) to protect all concerned with the trade industry thus ensuring its continuation.
The Nan Song was the first dynasty to allocate a standing army at a port of trade for protection. The reason for doing this was to protect all concerned with the trade industry thus ensuring its continuation.
This important dynasty controlled ancient China from 960 to 1279. The first thing that the dynasty accomplished was the reunification of the territory, divided since the Tang Dynasty was defeated. The Nan Song decided to protect their ports and territories from invasions. Trade was very important for the dynasty and did not want to risk this activity that represented good income for the Empire. The Khitans and the Tanguts frequently tried to invade Song's territories.