The objective of Jefferson's expedition of Lewis and Clark that is best represented in an illustration (assuming it depicts a landscape or geographical features) would likely be "gaining knowledge of the region’s geography." If the illustration focuses on the terrain, rivers, or specific geographical landmarks, this objective is the most appropriate response. If the illustration shows interactions with Native Americans or depicts their lifestyle, then "learning about the territory’s Native Americans" might be the correct answer. However, without seeing the illustration, it's difficult to provide a definitive answer.
Please let me know if you have more context or details about the illustration, and I can help you further!