The correct answer is: C: two and a half years. As a mater of fact, it started in May 1804 and lasted until September 1806. This expedition, also called Corps of Discovery Expedition was the first American expedition to cross what is now the western portion of the United States. Expedition was ordered by president Thomas Jefferson shortly after the Louisiana Purchase in 1803. The goals were to explore and map the newly acquired territory, find a practical route across the Western part of the continent, and establish an American presence in this territory.