Final answer:
Robert de La Salle extended France's influence in the Americas by exploring the Mississippi River and establishing trading posts on it, which paved the way for French claims and presence in the American interior.
Step-by-step explanation:
Robert de La Salle played a significant role in extending France's influence in the Americas. The correct answer to how he contributed is 'd. Explored the Mississippi River and built trading posts on it.'
La Salle is renowned for his expeditions in the late 17th century, particularly his 1682 expedition where he traveled down the Mississippi River to its mouth at the Gulf of Mexico. He claimed the entire Mississippi basin for France, naming it Louisiana after King Louis XIV. His voyages led to the establishment of French claims in the American interior, significantly extending French territorial claims beyond Canada.
La Salle’s exploration and his efforts in building trading posts along the Mississippi fostered trade, specifically in furs, and helped cement a French presence in what would eventually become the central part of the United States, long before the region fell into American hands through the Louisiana Purchase.