Answer: California, Nevada, Utah, Arizona, New Mexico, Colorado, and Wyoming.
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.
Another detail to add: The Mexican Cession reignited tension on the issue of slave-holding states vs. free states. The Missouri Compromise (in 1820) had admitted Missouri into the Union as a slave state with Maine being added at the same time to keep the balance of slave and free states equal. It also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state. Since that latitude line ran right through the middle of the Mexican Cession territory, there was bound to be further debate over the issue of slave vs. free states.
