Final answer:
The major river that flows from Minnesota to the Gulf of Mexico and contributed to the opening of the U.S. interior post-Louisiana Purchase is the Mississippi River, so the answer is A) Mississippi River.
Step-by-step explanation:
The major river that flows from its source in Minnesota to the Gulf of Mexico and helped open the interior of the U.S. after the Louisiana Purchase is the Mississippi River.
With Article VIII ensuring that Americans and British subjects could travel its entire length from source to ocean since 1783, the Mississippi has played a crucial role in U.S. history and commerce.
It was not until 1832 that the area of the headwaters of the Mississippi River was discovered and explored by non-Indians. The Mississippi River is a key feature of North America, helping to shape not only the landscape but also the historical progress of the country. Therefore, the correct answer to the student's question is A) Mississippi River.