Final answer:
Chen Xuanzang was likely selected to preside over the religious festival because his wisdom and religious stature could contribute to the legitimizing and unifying role of state-sponsored rituals, reflecting practices from the Qin and Han Dynasties.
Step-by-step explanation:
The selection of Chen Xuanzang to preside over the religious festival by the three county officials can be understood in the context of historical practices under the Qin and Han Dynasties, where religious figures and rituals played a significant role in legitimizing and exalting the emperor's power.
The historical documents from these periods, such as the annals of Shusun and Liu Bang's experience, show that sages, scholars, and religious leaders were entrusted with the organization and execution of state rituals that aimed to unify the empire and elevate the imperial authority. In this sense, a religious figure like Chen Xuanzang would be chosen for his wisdom, ability to unify people under a common spiritual experience, and likely for his capacity to contribute to the legitimacy of the rulers through ritualistic practices that symbolize the harmony and benevolence of the empire.