A cup of hot coffee, left on a table top, will go cold in few minutes. However, the same coffee can stay very hot, even for hours, inside an insulated thermos or a vacuum flask. Effectively, the insulated thermos prevents the heat loss to the surroundings.
There are 3 main mechanisms of heat transfer: conduction, convection and radiation. Conduction takes place through physical contact (and that is why a cup of hot coffee is hot to touch) and convection takes place through the movement of fluids (liquids or gases). Radiation, on the other hand, requires no physical contact.
An insulated thermos or vacuum flask is, in simple terms, a bottle inside another bottle, with vacuum separating the two. Since there is vacuum between the inner chamber and the outer casing of the thermos, no physical contact (or extremely little contact) takes place between the two and hence conduction and convection are almost non-existent. The radiation is also minimized to a great extent by the silver coating of the inside chamber in many of the thermoses available in the market today. However, there is still some heat transfer due to it.
Thus, among the given options, radiation (choice A) is the best choice.
Hope this helps.
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