Concept:The vacuum between the double walls of a thermoflask stops heat transfer that requires a material medium.
Explanation:Conduction and convection both need particles to transfer heat.
In a thermoflask, the space between the inner and outer walls is evacuated to create a vacuum.
Since vacuum has almost no air molecules, there is no medium for heat to travel through by conduction or convection.
Therefore, the vacuum specifically reduces heat loss due to conduction and convection.
Radiation, however, does not need a medium, so it is reduced separately by the silvered reflective walls, not by the vacuum.
Answer:B. conduction and convection only