The transfer of heat by convection involves the:

Convection is a mode of heat transfer that occurs in fluids, which can be gases or liquids. It relies on the movement of the fluid itself to transfer heat. As the fluid is heated, it becomes less dense and rises, while cooler fluid moves in to take its place, creating a circulation pattern. This movement of the fluid carries thermal energy from one place to another, allowing the heat transfer to occur.

In contrast, the reduction of mass pertains more to the quantity of material rather than the transfer of heat. Transfer of elements does not directly relate to the mechanics of heat transfer, as it may involve chemical reactions rather than thermal dynamics. Contact between molecules refers more to conduction, another heat transfer method, where energy is transferred through direct contact rather than through the motion of fluid. Therefore, the correct characterization of convection is indeed tied to the movement of a fluid.