In the context of the graph you've described, the element considered inflexible is typically AS (Aggregate Supply).
In economic models, Aggregate Supply can be thought of as relatively inflexible in the short run, especially if it is represented as upward sloping or if the increase is concave up. This indicates that prices may not adjust immediately or fully to changes in aggregate demand. Conversely, the Aggregate Demand (AD) curves can shift relatively more easily in response to changes in spending or other demand-side factors.
Therefore, the correct answer is AS.