The correct answer is - wave erosion.
The sea stack, sea arch, and the sea caves, are all land features that have gradually been formed by the influence of the waves, or rather by the erosion that the waves have caused on them.
The sea stack is a land form very close to the coast, and it is manifested by solitary very steep rocks, or as group of very steep rocks, and they have formed because of the wave and wind erosion.
The sea arch is land form that appears as an opening in a cliff, being formed by the the wave refraction where the focus of waves goes on the sides of a headland.
The sea caves are openings deeper into the crust, where the waves managed to erode the softer rocks, and the water to penetrate deeper inside.
