Which body of water connects Lake Ontario to the St. Lawrence River?

The Niagara River is the correct answer because it serves as the key waterway connecting Lake Ontario to the St. Lawrence River. The river flows northward from Lake Ontario and eventually joins the St. Lawrence River at Lake Ontario's upper end, near the town of Niagara-on-the-Lake. This connection is significant for navigation and water flow between these two major bodies of water.

While the Humber River, Trent-Severn Waterway, and Welland Canal are also important waterways in their own right, they do not serve the same purpose of connecting Lake Ontario directly to the St. Lawrence River. The Humber River primarily flows into Lake Ontario from the west, and the Trent-Severn Waterway functions as a canal system connecting Lake Ontario with the upper Great Lakes, but it does not provide a direct link to the St. Lawrence River. The Welland Canal, on the other hand, allows ships to bypass the Niagara Falls between Lake Erie and Lake Ontario, but it also does not connect directly to the St. Lawrence River. Thus, the Niagara River is uniquely positioned as the direct link.