Answer: A. Analyses of the behavior of seismic waves.
Explanation: "The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes" (Structure of the Earth). Combining the layered structure and reflection magnitudes with the magnitude of gravity (and therefore the average density of the Earth), we can get a pretty good idea of the density of each layer. By integrating density, we can calculate the pressures at depth."
