What is a seismic wave caused by?

A seismic wave is indeed caused by the sudden release of energy in the Earth's crust, which typically occurs during an earthquake. This release of energy generates vibrations that travel through the Earth, and these are what we refer to as seismic waves.

When we consider the correct answer, it highlights that an underwater earthquake—often occurring along tectonic plate boundaries—can generate powerful seismic waves that travel through the water and the surrounding earth, causing tremors felt on land. These underwater earthquakes are often associated with tectonic activity beneath the ocean floor, leading to significant seismic events.

In relation to other choices, while an earthquake on land can indeed produce seismic waves, the broader context and additional scope of the first option acknowledge that seismic waves can arise from various geological activities, including those that occur beneath the ocean. Volcanic eruptions and meteor impacts can also generate seismic waves, but the first option encompasses a more comprehensive understanding that includes major underwater seismic events which are known to affect larger areas and even trigger tsunamis. Thus, the emphasis on underwater earthquakes or volcanoes captures the essential nature of seismic wave generation.