Hey there!
The answer would be B. are drifting on a sea of magma.
The crust is broken up into giant tectonic plates that ride atop the magma, hot melted-rock material that makes up much of the interior of the Earth. Convection in the molten rock of Earth's mantle drives the movement of the plates. Hotter material rises to the surface, while cooler material sinks toward the core.
Hope this helps !
