The correct answer is:
A. Buddhist beliefs spread throughout China.
The reunification of China under Emperor Wen of the Sui Dynasty led to increased stability, which facilitated the spread of Buddhism throughout the region. Options B, C, and D do not accurately reflect the outcomes of the Sui Dynasty's establishment and the reunification of China. The Sui Dynasty was relatively short-lived, lasting only a short period, and did not restore the Han Dynasty. While the Sui Dynasty did undertake military campaigns, Korea was not conquered during that time.