A liquid remains hot or cold for a long time in thermos flask because there is no loss or gain of heat by:

Detailed Solution:

The correct answer is “Both (a) and (b)”.

A thermos flask minimizes heat transfer through all three methods:

1) Conduction: The double-walled construction with a vacuum between the walls prevents heat transfer through direct contact.

2) Convection: The vacuum between the walls also prevents heat transfer through the movement of fluids (convection).

3) Radiation: The reflective coating on the inner wall of the flask minimizes heat transfer through radiation, which is the transfer of heat through electromagnetic waves.