What was the key to American security during the nineteenth century?

A) the largest naval fleet in the world
B) troops stationed on the borders
C) geographical isolation
D) the largest nuclear arsenal in the world

Geographical isolation was the key to American security during the nineteenth century. The United States was separated from Europe by the Atlantic Ocean, which made it difficult for potential enemies to launch a direct attack on American soil. This isolation also gave the U.S. time to build up its military defenses and establish itself as a world power. The country did not have the largest naval fleet or nuclear arsenal during this time period, and troops stationed on the borders were not as essential due to the geographical barriers in place.