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

Correct answer: (c) Both (a) and (b)

Explanation

A thermos flask (Dewar flask, 1892) prevents heat transfer by all three modes.

The double wall and vacuum stop conduction and convection, while the silvered surfaces stop radiation.

Thus loss/gain occurs by neither conduction nor convection and radiation, keeping the liquid hot or cold for long.