Which of the following was true about the land between the Appalachians and the Mississippi in the 1800's?

You didn't say what type of event, which would narrow if down from many possible answers, but the fact that you put the question in Geography indicates you want something related to geography, so here are two possible events that happened there in 1800:

July 4 - Indiana Territory is formed by an Act of Congress as the first new territory created from the lands of the Northwest Territory.

July 10 - Connecticut cedes its Western Reserve (an area in present-day northeastern Ohio) to the federal government, which adds it to the Northwest Territory.

(No major earthquakes until 1811.)