The final borders of the lower 48 states were established in the

Question

a
Treaty of Guadalupe Hidalgo
b
Webster-Ashburton Treaty
c
annexation of Texas
d
Gadsden Purchase

Answer 1

The final borders of the lower 48 states were established in the d) Gadsden Purchase. This agreement, finalized in 1854, involved the U.S. purchasing land from Mexico to facilitate a southern transcontinental railroad and finalized the boundaries with Mexico.