Final answer:
The Paraná River is the longest river in Argentina and the 2nd longest in South America.
Step-by-step explanation:
The longest river in Argentina and 2nd longest in South America is the Paraná River.
It flows through Brazil, Paraguay, and Argentina, and eventually merges with the Uruguay River to form the Rio de la Plata.
The Paraná River stretches for about 2,549 miles (4,110 kilometers) from its headwaters in Brazil to its mouth in the Atlantic Ocean.