Final answer:
Buffer states have historically been used to maintain peace and prevent conflicts between larger powers; in the case of the Hunnu and Chinese states, it served to provide a zone of separation, fit into China's broader geopolitical strategy, and resulted in stabilized frontier relations.
Step-by-step explanation:
The purpose of a buffer state between the Hunnu (Xiongnu) and Chinese states was principally to maintain peace and prevent conflicts between the two states (Option 1). These buffer states provided a zone of separation to control and reduce the risk of invasions or military confrontations. The existence of buffer states was a crucial element of China's historical geopolitical strategy to secure its heartland, maintain the unity within the Han Chinese majority, and protect itself against both direct and indirect foreign influence and aggressions. Within the context of the relationship between the Han Dynasty and the Xiongnu, the Han Dynasty recognized the Xiongnu as equals and struck a marriage alliance, establishing tribute and political boundaries to ensure a stable frontier and preserve peace.