(A) Progressivism
(B) ** Imperialism
(C) Isolationism
(D) Defense Alliances
EXPLANATIONS BELOW
Concept note-1: -The Frontier thesis was formulated 1893, when American historian Frederick Jackson Turner theorized that the availability of unsettled land throughout much of American history was the most important factor determining national development.
Concept note-2: -But the most important effect of the frontier has been in the promotion of democracy here and in Europe. As has been indicated, the frontier is productive of individualism.
Concept note-3: -Turner argued that the frontier had made the United States unique. Due to hardship, residents were forced to become resourceful and self-reliant. They developed strength and “rugged individualism, ” which in turn fostered the development of democracy.
Concept note-4: -American frontier, in United States history, the advancing border that marked those lands that had been settled by Europeans. It is characterized by the westward movement of European settlers from their original settlements on the Atlantic coast (17th century) to the Far West (19th century).