The correct answer is A) The United States gained land from Mexico to build a planned rail.
An effect of the Gadsden Purchase was that the United States gained land from Mexico to build a planned rail.
The United States finally reached an agreement to pay Mexico $10 million for a portion of land. The agreement was called the Gadsen Purchase of 1854, in which the US acquired 29, 670 square miles that formed part of the New Mexico and Arizona territories. This land as much needed for the construction of a Transcontinental Railroad that connected the eastern part of the US with the Pacific Coast.