Final answer:
Argentina was the country that was first called the United Provinces of Rio de La Plata.
Step-by-step explanation:
The country that was first called the United Provinces of Rio de La Plata was Argentina. In March 1816, Argentina's conservative leaders declared its independence from Spain and proclaimed the United Provinces of the Rio de la Plata.
This declaration marked a practical recognition of their political situation rather than a sign of revolutionary militancy.