Which coastal landform is specifically shaped by the process of hydraulic action?

Hydraulic action is a significant coastal erosion process where the force of waves compresses air in cracks and crevices on cliffs and rocks. This pressure can cause portions of the rock to break off and erode over time. Hence, cliffs are specifically shaped by hydraulic action as the relentless force of waves drives this process, leading to the steep faces and features commonly observed along coastlines.

In contrast, the other choices do not primarily result from hydraulic action. Beaches are formed by the accumulation of sediment through wave action, sandbars are formed by the deposition of sand, and sand dunes result from the accumulation of sand, typically driven by wind, rather than direct hydraulic action. This makes cliffs the landform most closely associated with the erosive capabilities of hydraulic action.