Answer: the southwestern US.
Context/detail:
The Mexican Cession was territory ceded (meaning given up) to the United States in 1848 after the Mexican-American War. The territory included lands that represent the full states of California, Nevada, and Utah, as well as large portions of Arizona and New Mexico, and about a quarter of what would become the state of Colorado. There was also a small section of Wyoming involved. So that represents the bulk of territory that is in Southwest of the present United States.