Final answer:
Spain opened the Mississippi River to American shipping out of fear of an alliance between the United States and Britain.
Step-by-step explanation:
The correct answer is Spain feared an alliance between the United States and Britain. Spain was concerned that the United States and Britain would form an alliance and attack Spanish territories in the Americas. To prevent this alliance, Spain decided to open the Mississippi River to American shipping, in the hopes of cultivating good relations between the two countries. This decision allowed American traders to use the Mississippi River for transportation and trade, which helped facilitate economic growth in the western territories of the United States.
Learn more about Spain's decision to open the Mississippi River to American shipping