What allowed the united states to stay out of most "old world conflicts"?

I'd say location played a big part of it, depending how far you want to go back. 

During the 1930s, the combination of the Great Depression and the memory of tragic losses in World War I contributed to pushing American public opinion and policy toward isolationism. Isolationists advocated non-involvement in European and Asian conflicts and non-entanglement in international politics. Although the United States took measures to avoid political and military conflicts across the oceans, it continued to expand economically and protect its interests in Latin America. The leaders of the isolationist movement drew upon history to bolster their position. In his Farewell Address, President George Washington had advocated non-involvement in European wars and politics. For much of the nineteenth century, the expanse of the Atlantic and Pacific Oceans had made it possible for the United States to enjoy a kind of “free security” and remain largely detached from Old World conflicts. During World War I, however, President Woodrow Wilson made a case for U.S. intervention in the conflict and a U.S. interest in maintaining a peaceful world order. Nevertheless, the American experience in that war served to bolster the arguments of isolationists; they argued that marginal U.S. interests in that conflict did not justify the number of U.S. casualties.

Ithink the answer is d