Final answer:
During the Progressive Era, the U.S. expanded by shifting away from isolationism towards imperialism, increasing its military presence and acquiring new territories. The philosophy of imperialism and the desire for new markets and materials drove this change.
Step-by-step explanation:
The United States expanded during the Progressive Era primarily by becoming more engaged in international affairs and military matters, though the choices provided in the question do not directly describe how the country's expansion during this time was characterized. The Progressive Era saw America shift from isolationism to more of an expansionist mindset, influenced by the philosophy of imperialism. As the 19th century came to a close, the United States looked beyond its borders, engaging in imperialistic activities like the other great powers of the time. This involved increasing its military power, especially the navy, acquiring territories like Alaska and Hawaii, and looking for new markets and sources of raw materials.
The push toward becoming an international power and departing from earlier isolationism was based on a blend of economic motivations, strategic interests, and a belief in the cultural superiority often termed the 'White Man's Burden'. The protective tariff on imported goods and the expansion of the transportation infrastructure were internal policies that also supported economic growth, leading to a stronger national capacity for overseas expansion.