The correct answer is option 3: Radiation.

In thermos flasks, the silvering process is done to prevent heat transfer by radiation. Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation. The silvering in thermos flasks acts as a reflective surface that helps to reflect the infrared radiation back into the flask, effectively reducing heat transfer.

Option 1, convection, refers to the transfer of heat through the movement of fluids or gases. While convection can occur in thermos flasks, it is not the main mechanism by which heat transfer is reduced. The vacuum insulation present in thermos flasks minimizes convection by eliminating the presence of fluids or gases.

Option 2, conduction, refers to the transfer of heat through direct contact between objects. While conduction can occur in the components of the thermos flask, such as the inner and outer walls, the primary purpose of silvering is to reduce heat transfer by radiation, not conduction.

Option 4, both convection and conduction, is not correct because the main mechanism addressed by silvering in thermos flasks is radiation.