The correct answer is - a. earthquakes.
The earthquakes are unpredictable, and they can appear anywhere on the planet (most of them are on and around the plate boundaries though), so urbanizing the coastline doesn't increase or reduce the chances of this natural event.
The only consequence that can appear by an earthquake, on an urbanized coast, is if the earthquake appears in the water and causes a tsunami, but than again, it is something that is caused by the earthquake, not the direct impact of the earthquake itself.
