Final answer:
For walking on red-hot coals, dry feet are the best option to avoid bits of coal sticking and to minimize the risk of burns, as they won't undergo the harmful steam phase change and avoid added conductive materials.
Step-by-step explanation:
The question of whether bits of the coals do not stick to your feet while walking barefoot on red-hot coals concerns the physics of heat transfer and thermal conductivity. Assuming one has to walk on hot coals, the less heat transferred to the feet, the better. Given the options provided, having dry feet is the best choice. When your feet are wet, the water on your feet can quickly heat and turn into steam, which can cause burns. Oil might increase heat transfer due to its potential to have a higher thermal conductivity than the callus on your feet. Sand would provide a layer of insulation but could also retain the heat and cause burns as you walk. Dry feet will not undergo the steam phase change that wet feet will, and they will not have any added conductive materials like oil or sand that could retain heat. Plus, the callus on a practiced firewalker's foot provides a small layer of insulation that can resist heat transfer momentarily.