Final answer:
America's isolation until the late 1800s was primarily due to geographic barriers like the Atlantic Ocean and rugged terrain which made contact and expansion difficult. It was not due to America being landlocked or a lack of European interest.
Step-by-step explanation:
The best explanation for why America remained isolated until the late 1800s is D) Geographic barriers such as the Atlantic Ocean and rugged terrain limited contact. The vast Atlantic Ocean served as a significant barrier, making it difficult to maintain regular and close contact with Europe. Furthermore, the rugged interior terrain of the continent made exploration and expansion challenging.
It is also important to note that America's relative isolation was not due to a lack of European interest in colonization or Native American resistance, as Europeans had already established a presence in the Americas since the late 15th century. Additionally, America was not a landlocked continent, but rather bordered by two large oceans which eventually facilitated trade and expansion once maritime technology and naval power improved. Throughout the 19th century, the United States expanded across the continent, utilizing its natural resources and faced relatively little opposition from other foreign powers, thanks to its geographic advantages.