Convection currents in the earth’s mantle are responsible for the movement of tectonic plates. as shown in the diagram, heat from the earth’s core is the driving force that moves magma in a circular motion called "cells.” how is this circular motion achieved?

When the warmer parts go up and the colder parts go down in a circular motion. 

Answer;

hot magma rises and pushes up toward the lithosphere, and as it cools it cycles back down toward the core to be heated again.

explanation; convection currents are the circular pathways taken by a fluid that enhances heat transfer by the process known as convection which occurs in fluids.convection currents transfer heat from one place to another by mass motion of a fluid. earth mantle is in a semi-liquid state and behaves like any other fluid, rising up from the bottom of the mantle after becoming hotter and less dense from heat of the earth's core, as the rock loses heat into the earth's crust it becomes cooler and more dense, then sinking back down to the core, then repeating the cycle.

Centrapedial force is the answer