Answer:
C) The policy of establishing strong trade ties with the Yuan dynasty to deter a conquering force.
Step-by-step explanation:
Mongols use to rule by power. They were physically strong and had gained empire through brutality. Mongols wanted to rule Vietnam but they also wanted to increase trade stability. For this purpose Mongols decided to rule with peace. They had to change their nature and promoted peace during Pax Mongolica to flourish trade.