Scientists think this region may have once been a large sea plains

The correct answer is Interior Plains.

Explanation:

This region known as Interior Plains is a geographical region located close to the craton (the stable portion of the continental crust) of Central America. This region is delimited by the Gulf Coast region and the Arctic Ocean along the east flank of the Rocky Mountains.

in what process is there a multiple choice answer?

answer; continuing debate about how to interpret the constitution;