What mechanism drives mantle convection?

B is the correct answer because mantle convection is primarily driven by convection currents arising from the heat within the Earth's interior. The heat, generated by the decay of radioactive isotopes and residual heat from Earth's formation, leads to temperature differences within the mantle. These temperature differences create variations in density, causing warmer, less dense material to rise while cooler, denser material sinks. This cycle of rising and sinking creates convection currents that flow slowly through the mantle.

While other factors like the position of tectonic plates and geothermal gradients play roles in the dynamics of plate movement and thermal structure, they are not the primary mechanism driving the convection itself. Gravitational forces are important in the broader context of plate tectonics and earth structures but are not the central cause of the convection currents within the mantle. Thus, it is the convection currents generated by the heat from the Earth's interior that are fundamental to mantle convection.