What is radiation heat transfer primarily dependent on?

Radiation heat transfer is fundamentally based on the emission and absorption of electromagnetic waves, primarily infrared radiation, between surfaces that are not necessarily in contact and can be separated by a vacuum or a transparent medium. This process relies heavily on the concept of line-of-sight. The surfaces must have a direct line-of-sight between them to facilitate the transfer of radiant energy; otherwise, an obstacle or even just distance can inhibit the energy transfer.

In the context of the other options, touching surfaces directly relates to conductive heat transfer, where heat is passed from one object to another through direct contact. The movement of particles in liquids pertains to convective heat transfer, where heat is carried by the movement of fluids. The velocity of hot gases can influence convective heat transfer as well, but it does not apply to radiation transfer. Therefore, the correct answer emphasizes the importance of direct visual pathways between heat sources and receivers in the process of radiation heat transfer.