ANS:D - none of these.

Radiation heat transfer rates do not depend upon the distance between the heat source and the object receiving the heat. The rate of radiation heat transfer is determined by factors such as the temperature and surface area of the heat source, as well as the emissivity and temperature of the surfaces involved. However, unlike conduction and convection, radiation heat transfer is not significantly affected by the distance between the heat source and the object receiving the heat. This is because radiation propagates through electromagnetic waves, and its intensity diminishes with distance according to the inverse square law. Therefore, the correct answer is: distance between the heat source and the object receiving the heat.