What causes convection cells turn down?

since it is getting less dense it turns down and the denser fluid at the bottom turn up at the opposite side.

Since the cells are away from the heat source they cool. Cool cells are more dense causing those cells to sink. So the force is the increase in density.

it turns down c because it is like the oppiste way in a different way

ice cream

It turns because a convection cell is one big loop of a convection current