Which of the following is a key factor that influences seismic waves?

The correct answer is that density is a key factor that influences seismic waves. In geology and seismology, the properties of the materials that seismic waves travel through play a crucial role in how these waves propagate. Density, in particular, affects the speed of seismic waves; denser materials allow waves to travel faster than less dense materials. When seismic waves encounter different layers of the Earth, the varying densities can cause the waves to refract, reflect, or change speed.

Temperature and pressure can also impact seismic wave behavior; however, their effects are often intertwined with density changes in materials. While temperature can influence the physical state and properties of materials (for example, causing rocks to become more pliable or elastic when heated), it is density that provides a more direct and consistent influence on how seismic waves move through different geological layers. Humidity is not a relevant factor in the context of seismic waves, as it pertains more to atmospheric conditions rather than subsurface geological properties.