Question

when the mantle material is heated by the core, it rises towards the surface. as it rises, it cools, moves horizontally, and then becomes cooler and denser. finally, it sinks. this creates a cell.

Explanation:

The text describes the movement of mantle material due to heating from the core, rising, cooling, moving horizontally, sinking, which is the process of a convection cell. Convection cells involve the transfer of heat through fluid (or in this case, viscous mantle) movement, with warmer less dense material rising and cooler denser material sinking, creating a cyclic flow. So the word to fill in the blank is "convection".

Answer:

convection