Answer:
A. Treaty of Guadalupe Hidalgo.
Step-by-step explanation:
After the Mexican American War, the United States passed the Treaty of Guadalupe Hidalgo requiring Mexican Americans to prove that they legally owned land if they wanted to keep it.
The Treaty of Guadalupe Hidalgo, which brought an official end to the Mexican American War (1846-48), was signed on February 2, 1848, at Guadalupe Hidalgo, a city to which the Mexican government had fled with the advance of U.S. forces.