Final answer:
Heat flow within Earth changes its surface due to the cyclical movement of molten material that drives tectonic plates in different directions, a process explained by the theory of plate tectonics.
Step-by-step explanation:
The correct explanation for how heat flow within Earth changes its surface is that cyclical movement of molten material moves plates in different directions. Earth's lithospheric plates, which cover the molten iron core, are in constant motion due to the convection currents in the mantle. This process is driven by the heat escaping from the interior of the Earth, causing the upward flow of warmer material and the sinking of cooler material. These convection currents result in the tectonic plates moving apart at divergent boundaries, colliding at convergent boundaries, and sliding past each other at transform boundaries.
Plate tectonics is the theory that describes these movements, and it's essential for understanding how geological features like mountains and volcanoes are formed, as well as the occurrence of earthquakes. Additionally, the motion of plates at their boundaries can lead to subduction, where one plate slides underneath another. Overall, these movements reshape the Earth's surface over time.