A sea stack by far. This is because if you imagine a cliff face constantly being eroded away by the waves, the first sign of erosion to appear will be a crack in the rock which will soon open up more and become a cave. Eventually, the waves will keep eroding away the wall until it comes out the other side and becomes an arch. The roof will then collapse due to more erosion and leave a high stack of rock stranded in the ocean. This is the sea stack. ;)
