Final answer:
The desire for overseas markets and national security (option d) was a major factor in the shift toward American imperialism in the late 19th century.
Step-by-step explanation:
A major factor in the shift in American foreign policy toward imperialism in the late nineteenth century was the desire for overseas markets and national security(option d) . Driven by significant industrial and economic growth, the United States sought to expand its influence and control to ensure access to international markets and the raw materials needed for domestic industry. This era was characterized by the United States actively pursuing overseas territories, notably after the Spanish-American War in 1898, which resulted in the acquisition of the Philippines and heightened American presence in the Caribbean and Pacific. The closing of the American frontier was thought to necessitate new territories for economic expansion, and Alfred Mahan's ideas about the importance of naval power and strategic bases influenced America's approach to building an empire, though he did not believe the reopening of the American frontier was necessary.