The river situated along the border between the United States of America and Mexico is:

1. A

   The Mississippi

   Option A claims the Mississippi forms the US-Mexico boundary. This river flows entirely within the United States and does not mark the international border.

2. B

   The Rio Grande

   Option B claims the Rio Grande is situated along the border between the United States of America and Mexico, which is geographically correct and serves as a natural physical boundary in North America. This option correctly identifies the major river that separates Texas in the US from several northern states of Mexico, establishing it as the official key for this question.

3. C

   The Amazon

   Option C suggests the Amazon, but this massive river is located in South America, primarily flowing through Brazil and Peru far away from North America.

4. D

   The Colorado

   Option D names the Colorado River, which does cross into northwestern Mexico from the US, but it does not primarily form the main international border line like the Rio Grande.

Summary. Official key is (b) The Rio Grande. The question asks for the river that forms the border between the United States and Mexico. While the Colorado River touches both countries briefly, the Rio Grande serves as the principal natural boundary along this international frontier. The Mississippi and Amazon are located in entirely different regions and countries. Therefore, option (b) is the correct answer.