The correct answer is "nature".
The Westward Expansion was the period that comprises the immigration of settlers to the west. It was motivated by various reasons, including the political relocation of Native Americans to the territories west of the Mississippi River, the Gold Rush of California and the Mexican American War, which concluded with the annexation of the State of Texas to the United States.
Nature was a common romantic theme for art during the time. As explorers who went west wanted to show the rest about the landscapes of the territories found in the area. Portraying the search for freedom in their paintings.