Final answer:
The United States became an imperialist power to increase military strength, access new markets for its economy, and spread its culture, influenced by industrial, religious, and social motivations.
Step-by-step explanation:
The statement that best describes why the United States became an imperialist power is that it wanted to increase its military power, open new markets for its economy, and spread its culture. Early industrial expansion led to a need for new markets to export goods and secure raw materials. America's victory in the Spanish-American War and the desire for an 'open door' policy in China, for access to the vast Chinese market, exemplified this imperialistic approach. The economic, religious, and social interests alongside militarism created a stage where the U.S. began extending control over other nations through imperialism. Businesses sought out new markets, religious missionaries were driven to spread Christianity and American values, and the Industrial Revolution provided the economic and technological means for empire building.