The correct answer is option 1, Hudson Bay. The question asks which body of water is to the south-east of both Lake Ontario and the St. Lawrence River, which are located in the United States and Canada.

Let`s analyze the other options to better understand why they are not the correct answer:

Option 2, Labrador Sea, is to the northeast of both Lake Ontario and the St. Lawrence River. It is not located to the southeast as the question asks.

Option 3, Atlantic Ocean, is to the east of both Lake Ontario and the St. Lawrence River. While it is to the south of both bodies of water, it is not directly southeast.

Option 4, Newfoundland, is an island located to the east of both Lake Ontario and the St. Lawrence River. While it is to the southeast of them, it is not a body of water itself.

Therefore, the correct answer is option 1, Hudson Bay, which is located to the southeast of both Lake Ontario and the St. Lawrence River.