A thermos flask is a type of container that is designed to keep the temperature of its contents constant for as long as possible. It does this by preventing heat loss or gain through three different mechanisms: conduction, convection, and radiation. The flask consists of two walls with a vacuum between them, which minimizes heat transfer by conduction or convection. Additionally, the walls are coated with a reflective material to minimize heat transfer by radiation. Therefore, the correct answer is: "prevent heat loss or gain by conduction, convection, and radiation."