ANS:D - radiation

Heat transfer by: d) radiation may not necessarily require the presence of a medium. Radiation is the transfer of heat energy in the form of electromagnetic waves. Unlike conduction and convection, which require a medium (such as a solid, liquid, or gas) to transfer heat, radiation can occur through a vacuum or across empty space. This is because electromagnetic waves, including thermal radiation, can propagate through vacuum without the need for a material medium. Therefore, heat transfer by radiation can occur even in the absence of a medium.