One could make the argument for any one of the aforementioned events. However, I would have to argue that the Louisiana Purchase was the most significant for a few different reasons:
1) Jefferson used the concept of implied powers in order to buy this land from France. Nowhere in the US Constitution does it say that presidents can buy land from foreign countries. However, Jefferson argues that this power can be implied. Due to Jefferson's actions, several presidents after him will follow suit and buy territories from other countries.
2) It doubled the size of the United States.
3) It initiated the US's policy of Manifest Destiny. This concept that America should control all land from the Atlantic to Pacific Ocean was one that was introduced in the early 19th century. The first big land annexation of this time was the Louisiana territory (1803). After this purchase, the US would continue to grow in size thanks to buying land from other countries and war.
Answer:
C) it kept religious and political decisions separate
