In 1519, Spanish conquistador

Choose... (Francisco Pizarro, Hernán Cortés, Vasco Núñez de Balboa or Ferdinand Magellan)
sailed to the Americas. He landed in
Choose...(Mexico, Brazill, Argentina, or the Bahamas)
, where he made alliances with Aztec enemies. His alliances helped him conquer the powerful Aztec empire and take control of Mexico.

The answer is Hernán Cortés who sailed to the Americas in 1519 and landed in Mexico where he made alliances with Aztec enemies, eventually leading to the conquest of the Aztec Empire.

The Spanish conquistador who sailed to the Americas in 1519 and landed in Mexico, where he made alliances with Aztec enemies, is Hernán Cortés.

To arrive at this answer, we can eliminate the other options by considering historical facts. Vasco Núñez de Balboa was the first European to sight the Pacific Ocean, but his explorations were focused on Central and South America, not Mexico. Ferdinand Magellan is known for circumnavigating the globe between 1519 and 1522, but he did not make significant conquests in the Americas. Francisco Pizarro conquered the Inca Empire in South America, specifically what is now Peru. Therefore, the only option that matches the given criteria is Hernán Cortés.

The Spanish conquistador who sailed to the Americas in 1519 and made alliances with Aztec enemies to conquer the powerful Aztec empire and take control of Mexico was Hernán Cortés.