After the end of the Mexican War, the United States gained a large area in the West from Mexico. The Option C is correct.
At the end of the Mexican-American War (1846 to 1848), the United States emerged as the victor and signed the Treaty of Guadalupe Hidalgo with Mexico in 1848.
This treaty marked a turning point in U.S. history as it resulted in the acquisition of a vast territory in the West including present-day California, Nevada, Utah, Arizona, New Mexico and parts of Colorado, Wyoming, Kansas and Oklahoma.
In exchange, the United States paid Mexico $15 million and assumed around $3.25 million in Mexican debts owed to U.S. citizens. This acquisition often referred to as the Mexican Cession greatly expanded U.S. territory and as well had profound implications for the nation's future including the issue of slavery and the westward expansion.
The full question is:
Which of the following happened at the end of the Mexican War?
A. The United States gave up Texas to Mexico.
B. Borders returned to their prewar locations.
C. The United States gained a large area in the West from Mexico.