According to what I found, the answer is S America. You'd think it would be in Antartica--kind of a trick question, I think.
"Ushuaia, the southernmost city of the world, is the capital of the province of Tierra del Fuego, Antártida and South Atlantic Islands. Situated on the border of the Beagle Canal and surrounded by the Mounts Martial, this city offers a unique landscape in Argentina made up of mountains, sea, glaciers and woods."