Answer: Convection.
Explanation:
We have 3 types of heat transmission.
Radiation: Heat is transported as electromagnetic waves, this is not the case shown here.
Conduction: When we have two objects with different temperatures and we put them in contact, the system will want to go to equilibrium (both objects have the same temperature) so the hot object will give heat to the cooler object. We usually see this with solid objects.
Convection: It is the heat transfer due to the bulk movement of the molecules, so this need a material medium (it can not take place in solids, because the position of the molecules is fixed in the solids).
Then the rising of hot plasma and sinking of cooler plasma is convection.
