In the Earth's mantle, heat is transferred in large convection currents. Within these currents,

A. cooler and hotter rock rise but do not sink.

B. hotter and cooler rock sink but do not rise.

C. hotter rock rises and cooler rock sinks.

D. cooler rock rises and hotter rock sinks.

C. The hotter rock rises and the cooler rock sinks

Heat rises and as the warmer rock gets further away from the superheated center of the Earth it begins to cool off and starts to sink back toward the superheated center of the Earth where it is reheated and the whole process starts over again. That is what a convection current is.

heat from the core and the mantle itself causes convecton currents in the mantle.