The correct option is (d).
Explanation
A Tsunami (Japanese for "Harbour Wave") is a series of waves in a water body caused by the displacement of a large volume of water. Unlike normal ocean waves generated by wind, tsunamis are generated by geological forces that vertically displace the water column.
Analysis of Phenomena:
- 1. Volcanic eruption: Correct. Violent underwater volcanic eruptions or the collapse of volcanic islands into the sea can displace massive amounts of water, generating tsunami waves.
- 2. Underwater landslides: Correct. The movement of large masses of sediment or rock along the continental slope (often triggered by earthquakes) can displace water and create tsunamis.
- 3. Sudden vertical movement of the sea floor: Correct. This is the primary mechanism by which earthquakes generate tsunamis. When tectonic plates shift vertically (thrust faults), the overlying water column is pushed upwards, creating a wave.
- 4. Earthquake: Correct. Undersea earthquakes are the most common cause of tsunamis. However, usually, only earthquakes that cause vertical displacement of the seabed (not strike-slip faults) generate significant tsunamis.
Key Takeaway:
Any geological event that causes a sudden and massive vertical displacement of the ocean water column can trigger a tsunami. This distinguishes them from tidal waves, which are caused by gravitational forces.