The builder of a house on a cliff overlooking the ocean should be concerned about whatgeologic phenomenon?

Erosion

Explanation:

Building a house on a cliff is a very trick thing. The cliffs usually are not composed of hard rocks, but instead are usually composed of sedimentary rocks which are much softer and less stable on the long run. The cliffs are also exposed to heavy erosion, in the lower parts because of the waves, in the higher parts because of the winds. The erosion makes the cliffs slowly fall apart, and occasionally even crumble down, so a builder that would build a house on a cliff should be wary of all these things and construct the house where it is the safest for the long run.