The long-run average total cost curve is C-shaped.
This shape reflects the typical behavior of average total costs as output increases. Initially, as production increases, average total costs tend to decrease due to economies of scale. However, after reaching a certain point, average total costs may begin to rise due to diseconomies of scale. So, the C-shaped curve illustrates this relationship effectively.
The other choices provided do not accurately represent what the long-run average total cost curve demonstrates.