The natural boundary that helps to form the border between the United States and Mexico is Rio Grande.
<h3>What is known as
Rio Grande?</h3>
It is the name of big river that is located in North America. The Rio Grande flows from from the SW Colorado through central New Mexico and along the boundary between Texas and Mexico into the Gulf of Mexico. 1,800 miles long.
Hence, the natural boundary that helps to form the border between the United States and Mexico is Rio Grande.
Therefore, the Option D is correct.
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