Which event signifies the end of the frontier period in the United States according to historical contexts?

The closing of the frontier in 1890 is widely recognized as a pivotal moment that marked the end of the frontier period in the United States. The U.S. Census Bureau declared the frontier officially "closed" after determining that there was no longer a discernible line of settlement that could be considered as free land for those seeking new opportunities. This closure indicated a significant transformation in American society and culture, as the availability of land for new settlers had been a driving force behind the nation’s expansion and development.

The closing of the frontier emphasized the transition from an era characterized by westward expansion and the notion of unlimited opportunities to a realization that the United States had fully settled its available land. This event also sparked debates about the implications for American identity and growth, spawning reflections on issues such as urbanization, industrialization, and the evolving nature of social and economic opportunities in a rapidly modernizing nation.

In contrast, other events listed, like World War I or the establishment of the Homestead Act, do not directly denote the conclusion of an expansive frontier period. While each of these played a critical role in shaping modern America, they do not correlate with the specific demarcation of the frontier era as the census findings in 1890 do. The