One reason the United States claimed Oregon Country was because of A. the 1844 presidential election.

B. the Lewis and Clark expedition.

C. the Treaty of Guadalupe.

D. the Whitman massacre.​

B) The Lewis and Clark expedition

Explanation:

According to Wikipedia:

"The U.S.-based its claim in part on Robert Gray's entry of the Columbia River in 1792 and the Lewis and Clark Expedition. Great Britain based its claim in part on British overland explorations of the Columbia River by David Thompson and on prior discovery and exploration along the Coast."