Which river forms part of the eastern border of North Dakota?

The Red River forms part of the eastern border of North Dakota and serves as a significant geographical feature separating North Dakota from Minnesota. This river flows northward into Canada and is an important waterway in the region, historically serving as a trade route and a natural boundary. The Red River's drainage basin also plays a crucial role in the ecology and economy of the surrounding areas.

Other rivers listed in the question do not serve this specific purpose. The Missouri River runs through the state but does not form the eastern border; rather, it flows along the western part of North Dakota. The Yellowstone River primarily is located in Montana and flows eastward, while the Sheyenne River, although it runs through North Dakota, does not define the eastern border of the state. Thus, the Red River is the correct choice as it serves the specific geographical function described in the question.