Final answer:
The city of Buenos Aires is located on the western shore of the Rio de la Plata estuary and played a crucial role as a strategic port in the Spanish colonial economy.
Step-by-step explanation:
The city that lies on the western shore of the Rio de la Plata estuary is Buenos Aires. Buenos Aires became the capital of the Viceroyalty of Río de la Plata in 1776, which included the territories of modern-day Argentina, Bolivia, Paraguay, and Uruguay.
As an important port, it served as the main outlet for goods like silver from Bolivia and agricultural products from the region.
Buenos Aires has historically been a hub of economic activity and resistance, which eventually contributed to the surrounding regions' push for independence from Spanish colonial rule.