The correct answer is b. - tsunamis.
Tsunami is a Japanese word meaning harbor wave.
Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. This usually happens deep out in the ocean. The earth movement causes a displacement of water in the form of a large ripple which begins to move inland at tremendous speed.
There is no dramatic increase in height of the wave, but as it moves toward the coast, it builds up to a higher and higher height as the ocean becomes shallower and shallower till when it makes land fall, the wave may be dozens of feet high.
