Which French explorer is known for exploring the St. Lawrence Seaway?

Jacques Cartier is recognized for exploring the St. Lawrence Seaway, as he was the first European to map and navigate this vital waterway in North America during his voyages in the early 16th century. Cartier's expeditions in 1534, 1535, and 1541 led him to the St. Lawrence River, where he established contact with indigenous peoples and sought to understand and document the region's geography, resources, and potential for colonization. His explorations were instrumental in paving the way for future French settlement and claim in North America.

The other figures listed in the options are not associated with the St. Lawrence Seaway. Samuel de Champlain, while an important French explorer, is known for founding Quebec and further exploring the Great Lakes, not the initial exploration of the St. Lawrence. Hernán Cortés was a Spanish conquistador famous for his conquest of the Aztec Empire in present-day Mexico, and Ferdinand Magellan was known for leading the first expedition to circumnavigate the globe, which did not involve exploring North America. Thus, Cartier stands out as the principal explorer of the St. Lawrence Seaway.