Actually, the answer is B "the development of the hydrogen bomb" :)
The United States emerged as a great industrial power following World War I -- the most powerful nation in the world, in fact.
The growth of the United States as the world's leader in industry had been proceeding rapidly already prior to the Great War (which we know as World War I). By 1900, 38% of the world's wealth was held by the United States. By 1914, the US produced as much coal as Britain and Germany combined, as well as producing over 40% of the world's iron.
But before World War I, the United States tended to take an isolationist stance toward other nations. World War I advanced the US into superpower status as a nation that used its industrial might to involve itself in global affairs.
