What was meant by the "closing" of the Western frontier? the settlement of most of the land between the East and West Coasts
the failure of American railroad companies to establish cities and towns out west
the evacuation of many Western towns due to attacks from Native Americans
the decision by the United States government to prevent additional settlement

In 1890, the Census Bureau broadcast the closure of the frontier, meaning that in the west there was no apparent tracts of land without settlers. This news was a distinguished event in American history; the frontier represented danger because of the Natives who lived in the region but also freedom and opportunity.

Explanation:

b. the colonies must seek protection from other nations.