Final answer:
By the end of the 19th century, the U.S. controlled the Philippines, Guam, Puerto Rico, and had influence over Cuba after the Spanish-American War, marking an era of American imperialism.
Step-by-step explanation:
By the end of the nineteenth century, the United States had expanded beyond its continental borders and controlled several overseas possessions. The correct answer to the question is a) Philippines, Guam, Puerto Rico, Cuba. These territories came under U.S. control after the Spanish-American War of 1898. While Cuba eventually became independent in 1902, it remained under U.S. influence for many years thereafter.
The Philippines also gained independence later, whereas Puerto Rico and Guam are still U.S. territories today. This expansion into overseas colonialism marked a significant change in American foreign policy known as imperialism, where the U.S. joined other industrialized nations in acquiring territories for strategic and economic purposes.