ANS:B - mercury

Mercury is not commonly used as a medium for high-temperature heating. While it has a relatively high boiling point for a liquid, it is not typically used as a heat transfer fluid for high-temperature applications due to its high toxicity and potential environmental hazards. Dowtherm, liquid metal (such as molten sodium), and fused salts (eutectic mixtures) are all used as mediums for high-temperature heating in various industrial processes. These fluids are chosen for their ability to withstand high temperatures and their compatibility with the materials used in high-temperature systems. Therefore, the correct answer is: mercury.