The correct answer is:The Gadsden Purchase
Gadsden Purchase is a region of 76 845 kmĀ² of present-day southern Arizona and southwestern New Mexico that was purchased from Mexico by the United States in a treaty signed by US President Franklin Pierce who signed it on June 24, 1853 and the Mexican ruler Antonio Lopez de Santa Anna who signed it on December 30, 1853 and ratified by the United States Senate on April 25, 1854. The purchase included land south of the Gila River and west of the Rio Grande and was made for the purpose of building a transcontinental railroad along the southern route of the United States. It also solved the pending border problems after the Treaty of Guadalupe-Hidalgo that ended the Mexican-American War.