The correct answer is [C] The double walls of the shiny inner wall and the outer cover prevent heat from escaping or entering. A thermos flask is designed to minimize heat transfer and maintain the temperature of its contents. The key feature is the vacuum between the double walls, which effectively prevents heat transfer by conduction and convection since there are no molecules in the vacuum to transfer heat. Additionally, the inner walls are shiny and reflective, which reduces heat loss through radiation by reflecting infrared radiation back into the liquid. The outer cover adds an extra layer of insulation, further minimizing any potential heat exchange with the external environment. Together, these design elements work synergistically to ensure that the liquid inside the flask remains hot or cold for an extended period, making the thermos flask an efficient and practical solution for temperature maintenance.