Final answer:
The Missouri River is the tributary that contributes the most water to the Mississippi River, more so than the Ohio River, St. Lawrence River, or Rio Grande River.
Step-by-step explanation:
The tributary that adds the greatest volume of water to the Mississippi River is the Missouri River. The Missouri River is a major river that runs through the central United States and is actually longer than the Mississippi River itself. It has historically been important for commerce and exploration, especially as noted by the mission to explore its connection to potential water routes to the Pacific Ocean. The Missouri River becomes a tributary of the Mississippi River at St. Louis, Missouri, contributing a significant volume of water to the Mississippi, more than any other tributary including the Ohio River, St. Lawrence River, and Rio Grande River.
Unlike the Missouri River, the St. Lawrence River is not a tributary of the Mississippi River. It provides a connection between the Great Lakes and the Atlantic Ocean. The Rio Grande River, while a significant river, is located in the southwest of the United States and also not a tributary of the Mississippi. Lastly, the Ohio River is a significant tributary but does not contribute as much volume to the Mississippi as the Missouri River does.