Answer:
The correct answer is A. seize Mexican land south of the Rio Grande.
The Gadsden Purchase was a treaty signed between the United States and Mexico in 1853, which allowed the United States to purchase a strip of land in present-day Arizona and New Mexico for $10 million. The purpose of the purchase was to acquire land for the construction of a southern transcontinental railroad, which would connect the eastern and western coasts of the United States.
The land acquired in the Gadsden Purchase was located south of the Rio Grande river and was primarily inhabited by Mexican citizens. The acquisition of this land allowed the United States to expand its territory further south and west and to secure a vital route for the transcontinental railroad. The Gadsden Purchase was one of several significant land acquisitions made by the United States in the 19th century that contributed to the country's growth and expansion.