Which feature indicates the shift between hilly terrain and flat plains?

The fall line is the correct answer because it represents a geographical boundary where hilly or mountainous terrain transitions into flat plains. This feature often marks the point where rivers descend from higher elevations to lower plains, creating waterfalls and rapids, and can serve as a significant indicator in terms of natural resources and settlement patterns. Many cities were historically established along the fall line due to the advantage of water power for mills and favorable transportation routes.

The other options do not describe this specific transition. A pothole refers to a depression in the road or a small body of water formed by erosion; a panhandle typically refers to a narrow strip of land that extends from a larger area, indicating a specific shape rather than a terrain transition; and an archipelago is a group of islands, which does not pertain to the shift between hilly and flat areas at all.