Name the river that forms a major portion of the border between Texas and Mexico and flows past El Paso, Laredo, and Brownsville into the Gulf of Mexico.

• A. Mississippi River
• B. Colorado River
• C. Rio Grande (River)
• D. Pecos River

Answer: (C)

The river that forms a large portion of the Texas–Mexico border and flows past El Paso, Laredo, and Brownsville into the Gulf of Mexico is the Rio Grande. It starts in the mountains of Colorado, travels south through New Mexico, and then serves as the boundary between Texas and Mexico from the El Paso area toward the Gulf, passing near Laredo and finally reaching the Gulf at Brownsville. This path matches all the clues: it lies along the Texas–Mexico border and empties into the Gulf.

The other rivers don’t fit these details. The Mississippi River runs along the eastern edge of the United States and doesn’t form the Texas–Mexico border. The Colorado River in its popular sense flows through the southwestern U.S. and into the Gulf of California, not the Gulf of Mexico or past these Texas border towns. The Pecos River crosses Texas and New Mexico and feeds into the Rio Grande rather than forming the border with Mexico.