The answer would be the second option, the Gadsden Purchase. It refers to when the United States bought about 30,000 square miles of land in present-day New Mexico and Arizona from Mexico for $10 million dollars. It was negotiated by James Gadsden, the US Minister to Mexico. The land that was obtained provided a viable route for a southern transcontinental railroad from El Paso, in the far west of Texas, to Los Angeles, California on the Pacific coast. Hope this helps!
