The term of the Treaty of Paris 1783 that was a direct response to an unpopular British law which helped begin the American Revolution is:
C. The western border of the United States would be the Mississippi River instead of the Appalachian Mountains.
This change was significant as it addressed grievances related to British policies such as the Proclamation of 1763, which restricted colonial expansion westward beyond the Appalachian Mountains. This restriction fueled colonial discontent and contributed to the desire for independence.