Final answer:
Missouri is the state with St. Louis as its second largest city, and it includes the largest man-made monument in the U.S., the Gateway Arch, as well as other key cities, Springfield and Independence.
Step-by-step explanation:
The state has the second largest city St. Louis, known for having the largest man-made monument in the U.S.—the Gateway Arch, and also includes cities like Springfield (the third largest city in the state) and Independence, Missouri.
The Gateway Arch reflects St. Louis's role as the gateway to the West for pioneers and marks its history as a bustling trading and economic center. The importance of the state and its cities can be traced back to historical roots, such as the Missouri Territory being a significant part of the Louisiana Territory and a pivotal area in the sectional crisis that involved debates over the institution of slavery during Missouri's admission into the Union.
Moreover, St. Louis's vicinity to the ancient city of Cahokia signifies its longstanding significance as an urban and trade hub adjacent to significant indigenous cultural centers.