Final answer:
The Lewis and Clark Expedition primarily encountered Native American tribes during their journey across the Louisiana Purchase.
Step-by-step explanation:
The Lewis and Clark Expedition, led by Meriwether Lewis and William Clark, encountered various groups and challenges during their journey to explore the Louisiana Purchase. During their expedition from 1804 to 1806, they met and established relationships with numerous Native American tribes. These interactions were pivotal, as Thomas Jefferson's goals included mapping the territory, assessing its natural resources, and initiating trade with the Native populations. Given the options provided, the correct choice is A) Native American tribes.