I think the correct answer would be the first option. The process that would explain this phenomenon is convection. It is a mode of heat transfer which is a result of bulk movement of a substance due to the difference in density which is a result of the temperature of these substances. This transfer of heat is applicable to fluids. When the temperature of a fluid increases, the fluid expand thus decreasing its density. However, for a fluid which has a cooler temperature, it would contract resulting to a much higher density. Therefore, the warm fluid would rise up while the cooler fluid would sink below the warm fluid.