The correct answer is [B] To prevent heat loss due to radiation. The shiny inner walls of a thermos flask are designed to reflect infrared radiation, which is a form of heat transfer. By having a reflective surface, the flask minimizes the amount of heat that can escape through radiation. This is crucial because, even with a vacuum between the inner and outer walls to prevent conduction and convection, radiation can still occur. The reflective coating typically made of materials like aluminum, bounces the infrared rays back into the liquid, thereby maintaining its temperature. This design effectively traps heat inside the flask if the contents are hot or prevents external heat from entering if the contents are cold. This comprehensive approach to reducing heat loss ensures that the thermos flask can keep beverages at their desired temperature for extended periods, making it highly efficient and practical for everyday use.