Final answer:
The farthest south major city in South America is Ushuaia, located in Argentina, and it is considered the southernmost city in the world.
Step-by-step explanation:
The farthest south major city in South America is Ushuaia, which is located in Argentina. It is often referred to as the southernmost city in the world, positioned on the Tierra del Fuego archipelago. While other cities like Buenos Aires have historical significance and are well-known, Ushuaia holds the distinction of being the most southern major settlement. Cities like Rio de Janeiro, São Paulo, and Porto Alegre, while located in the southern part of South America, are not as far south as Ushuaia.