Final answer:
Convection currents best describe the movement of the mantle as tectonic plates slide around on its surface, causing various geological events and features.
Step-by-step explanation:
The best description of the movement of the mantle as the plates slide around on its top layer is A. Convection currents. These currents within the Earth's mantle drive the movement of tectonic plates, which can result in various geological phenomena, such as earthquakes, continental drift, mountain building, volcanic eruptions, and even the creation of the Hawaiian island chain. The tectonic plates may move apart, collide (where one can burrow under another - a process known as subduction), slide alongside each other, or jam together, dramatically altering the Earth's crust over time.