Which observation provides the strongest evidence for the inference that convection cells exist with the earths mantle?

The movement of tectonic plates on Earth's surface and the associated patterns of earthquakes and volcanic activity suggest the presence of convection cells in the mantle. These processes are driven by heat flow from the Earth's core, causing material to rise, cool, and sink in a cyclical pattern that is characteristic of convection cells. Additionally, seismic data showing variations in temperature and density within the mantle further support the existence of such cells.