Final answer:
Chile is the South American country located furthest west, as it lies along the Pacific coast and to the west of Argentina, Brazil, and Peru.
Step-by-step explanation:
The country in South America that is located furthest west is Chile. While all the options provided, which include Argentina, Brazil, Chile, and Peru, are western countries on the continent of South America, Chile extends the most westerly. This is primarily because the Andes Mountains span the entire western border of Argentina with Chile, positioning Chile along the Pacific coast to the west of Argentina, and southwest of both Brazil and Peru.