He tract of land the United States received at the end of the Mexican-American War in 1848 was called

the Mexican Cession.

the Republic of Texas.

the Gadsden Purchase.

the Louisiana Purchase.

The answer is: the Mexican Cession.

After the agreement of the Treaty of Guadalupe Hidalgo, which ended the Mexican-American war in 1848, the United States gained the disputed areas of Alta California and Santa Fe de Nuevo Mexico, called the Mexican Cession.

The rest of the options are incorrect because the region had been previously demanded by the Republic of Texas; the Gadsden Purchase refers to the territory of Arizona and New Mexico bought to Mexico by America in 1854; and the Louisiana Purchase was the acquisition from France of the Louisiana territory of New France in 1803.