Final answer:
The Gadsden Purchase in 1853 rounded out the present boundaries of the United States, except for Alaska and Hawaii. It was bought to ensure a direct line across the United States' territory for building a southern railroad to the Pacific.
Step-by-step explanation:
The subject of the question is History. The Gadsden Purchase, which occurred in 1853, is the correct answer. James Gadsden, the United States minister in Mexico, purchased 45,000 square miles of Mexican territory south of the Gila River for $10,000,000. This land was bought to ensure a direct line across the United States' territory for building a southern railroad to the Pacific. With this purchase, the cry for Manifest Destiny ended for a time.