Who was the first Spaniard to begin explorations of the mainland of North America?

The first Spaniard to begin explorations of the mainland of North America was Ponce de Leon. In 1513, he landed on what is now known as Florida, claiming the territory for Spain. His expedition is notably recognized for the search for the Fountain of Youth, a legendary spring said to restore the youth of anyone who drank from it. Ponce de Leon's exploration marked a significant moment in the Spanish colonization efforts in the New World and opened the door for subsequent Spanish expeditions into North America.

Other figures mentioned, such as Cortez, Columbus, and Magellan, made significant contributions to exploration, but they were focused on different regions and contexts. Cortez is famous for his conquest of the Aztecs in Mexico, Columbus is known for his voyages that led to the discovery of the Caribbean islands, and Magellan is celebrated for his circumnavigation of the globe. None of these explorers directly began the exploration of the North American mainland, making Ponce de Leon the correct answer.