What major geographical feature marks the southern boundary of the coastal plains?

The correct answer is the Fall Line. The Fall Line is a significant geographical feature that acts as a natural boundary between the coastal plains and the more elevated regions inland. It is characterized by a series of waterfalls and rapids where rivers descend from the hard rocks of the piedmont region to the softer sediments of the coastal plains. This transition marks a change in geography, climate, and ecology, serving as an important dividing line that has historically influenced settlement patterns, economic activities, and transportation routes in Georgia and the surrounding region.

The other choices represent well-known geographical features, but they do not serve as the southern boundary of the coastal plains. The Mississippi River is primarily associated with the western borders of states and is not relevant to Georgia's coastal plains. The Atlantic Ocean, while it does border the coastal plains to the southeast, does not mark their southern boundary. The Appalachian Mountains are located to the north and northwestern parts of Georgia and do not directly relate to the southern aspect of the coastal plains either. The Fall Line is thus the most accurate representation of the boundary in question.