Chang'an can best be described as

a major trading outpost along the Silk Road

China's leading agricultural region.

a flourishing cultural and commercial center.

a small but organized government center.

All of the options could partially describe Chang'an at different points in its history, but the most accurate and comprehensive description would be "a flourishing cultural and commercial center." Chang'an was the capital of multiple Chinese dynasties, including the Han, Sui, and Tang dynasties, and during these periods it thrived as a political, economic, and cultural hub. It was a cosmopolitan city that attracted scholars, traders, and artists from various regions, and it played a significant role in the exchange of ideas, goods, and technologies along the Silk Road. Therefore, "a flourishing cultural and commercial center" is the most fitting description for Chang'an.