The tool that has allowed scientists to learn about the characteristics of Earth's layers is the seismograph.
The tool that has allowed scientists to learn about the characteristics of Earth's layers is a seismograph. A seismograph is an instrument that measures and records the vibrations or seismic waves caused by earthquakes or other sources. By studying the patterns and characteristics of these seismic waves, scientists can infer information about the different layers of the Earth. Some examples of the characteristics they can learn about include the thickness, density, and composition of each layer. Therefore, the correct answer is "seismograph".
