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

A liquid remains hot or cold for a long time in a thermos flask because there is no loss or gain of heat by Conduction, Convection and radiation.

Key Points

The thermos flask consists of a double-walled glass vessel resting on a cork inside a metal or plastic case.

The insulating stopper or cork at the top minimizes heat loss by convection.

The outer surface of its inner wall and the inner surface of its outer wall are silvered which prevents heat transfer due to radiation.

The double-walled glass vessel and vacuum prevents heat loss by conduction.

The thermos flask was invented by Sir James Dewar in 1892.

Additional Information

Conduction:
In this process, heat is transferred from one place to another place by the successive vibrations of the particles of the medium without bodily movement of the particles of the medium.

In solids, heat transfer takes place by conduction.

Convection:
In this process, heat is transferred by the actual movement of particles from one place to another place.

Due to the movement of particles, a current of particles sets up, which is called a convection current.

Radiation:
In this method, the transfer of heat takes place with the speed of light without affecting the intervening medium.