Which river serves as the outlet for the Great Lakes to drain into the Atlantic Ocean?

The St. Lawrence River is the correct choice because it acts as the primary outlet for the Great Lakes, allowing water to flow from the lakes into the Atlantic Ocean. This river connects Lake Ontario to the Gulf of St. Lawrence, which is part of the larger Atlantic Ocean system. The flow of water through the St. Lawrence River is crucial not only for maintaining the water levels in the Great Lakes but also for facilitating navigation and transportation.

The other rivers listed do not serve this function. The Mississippi River primarily drains the central part of the United States into the Gulf of Mexico, while the Ohio River is a tributary of the Mississippi and is mainly associated with drainage in the southeastern and midwestern regions of the country. The Missouri River also flows into the Mississippi River, further complicating its relationship with the Great Lakes basin. Thus, the St. Lawrence River uniquely fulfills the role of connecting the Great Lakes to the Atlantic.