Final answer:
Jefferson's policies, including the sale of western lands in affordable lots and the Louisiana Purchase, were pivotal in encouraging western expansion, in line with his vision of an agrarian republic and increased trade in the West.
Step-by-step explanation:
Thomas Jefferson's policies as president that encouraged western expansion include his promotion of agriculture through the sale of western public lands in small and affordable lots. This policy was in line with his vision of an "empire of liberty", which aimed to create a rural republic of land-owning White men, or yeoman republican farmers. Furthermore, Jefferson's most significant contribution to western expansion was the Louisiana Purchase, which doubled the size of the United States and provided a substantial amount of land to the west for future settlement and development. Jefferson also looked to bolster trade in the West, understanding the importance of the port of New Orleans and the Mississippi River for American agricultural commerce.