Final answer:
Seismic wave detection has most significantly contributed to our understanding of the Earth's mantle and core, surpassing other methods by providing a deeper and more complete understanding of Earth's internal structures.
Step-by-step explanation:
The Earth's mantle and core understanding has most significantly been advanced by D) Seismic wave detection. Seismic waves, generated by natural processes such as earthquakes, allow scientists to study indirect evidence about the structure of Earth's interior. These waves change in velocity as they pass through different layers, and through the analysis of the paths, velocities, and refractions, geologists can infer the properties and composition of the Earth's mantle and core. The development of seismic reflection and refraction methods has been the key to mapping the internal structure of our planet. Volcanic activity provides direct evidence of mantle materials but does not give a complete picture of the earth's interior layers, making seismic studies more central to our understanding.