The correct answer is "recording vital information about the American West during their travels."
The most important accomplishment of Lewis and Clark was the recording of vital information about the American West during their travels.
Meriwether Lewis and William Clark explored the Louisiana Territory and kept detailed records of their findings.
After the Louisiana Purchase of 1803, US President Thomas Jefferson ordered the exploration of the unknown territories. The expedition was led by Meriwether Lewis and William Clark. The expedition started in 1804 and ended in 1806 when both explorers returned to Washington DC to inform President Jefferson of the results of the expedition.