Which type of heat transfer does not require a medium?

Radiation is the correct answer because it is the only form of heat transfer that does not require a medium to occur. It involves the transfer of energy through electromagnetic waves, such as infrared radiation, which can travel through a vacuum, unlike conduction and convection.

Conduction transfers heat through direct contact between materials, requiring a solid medium, while convection involves the movement of fluids (liquids or gases) and relies on a flow of material to carry heat. Advection, a specific type of convection, refers to the horizontal movement of air or water that carries heat with it, thus also dependent on a medium.

Understanding that radiation does not need a substance to transfer energy helps clarify key distinctions in the mechanisms of heat transfer.