Concept:A vacuum flask reduces heat transfer by using a vacuum to stop conduction and convection, and silvered walls to stop radiation.
Explanation:The vacuum between the double walls contains no air molecules.
Without air particles, heat cannot be transferred by conduction or convection through the vacuum.
Thus, the vacuum prevents conduction and convection.
The inner surfaces of the flask are silvered to make them shiny.
Shiny surfaces reflect thermal radiation back into the flask.
Therefore, the silvered walls minimise radiation losses.
Answer:Option B: prevent conduction and convection and minimise radiation.