From what time period did the first emperor of China unify kingdoms?

The first emperor of China, Qin Shi Huang, unified the various warring states of China in 221 BCE. This unification marked the beginning of the Qin Dynasty and the establishment of a centralized imperial system, which laid the foundation for subsequent Chinese dynasties and governance that followed. The significance of this event is profound, as it not only ended centuries of conflict between the states but also initiated significant reforms in administration, legal code, and standardization across the newly united China.

The other time periods listed do not correspond to the unification of China's kingdoms. The date 100 BCE falls within the Han Dynasty, which was the second imperial dynasty of China that came after the Qin Dynasty. The dates 500 CE and 1200 CE represent periods in which China was experiencing significant developments and changes, such as the Southern and Northern Dynasties and the Song Dynasty, respectively. However, none of these periods relate to the actual unification of the kingdoms under a single emperor, which specifically occurred in 221 BCE.