What does radiation refer to in the context of heat transfer?

In the context of heat transfer, radiation refers to the transfer of heat without requiring a medium, such as air or another substance, between two bodies. This process occurs because all objects emit thermal radiation in the form of electromagnetic waves, primarily in the infrared spectrum. When one body absorbs this radiation, it increases in temperature, even if there is no air or any other material between it and the source of the radiation.

Radiation is distinct from other forms of heat transfer, such as conduction and convection. Conduction involves the transfer of heat through direct contact between materials, while convection involves the movement of fluids (such as air or water) to carry heat. Neither of these processes requires the absence of a medium, making radiation unique in that it can occur through the vacuum of space, as seen with the heat from the sun reaching the Earth. This characteristic is what makes option C the correct answer in this context.