The United States acquired a significant amount of territory through the Mexican-American War, which took place between 1846 and 1848. This acquisition is known as the Mexican Cession, which included present-day California, Nevada, Utah, Arizona, and parts of Colorado and New Mexico, among other areas.
To determine which labeled territory (A, B, C, or D) corresponds to this acquisition, I would need additional information about what each territory is or a description of their locations on the map you mentioned. If you can provide details about what each labeled territory represents, I can help identify the correct one that corresponds to the United States' acquisition from the Mexican-American War.