Answer:
the gas molecules slow down, and some change phase to liquid.
Explanation:
1) The heat always travels from the warmer substance to the coleer substace.
That is why it is correctly indicated that the gas molecules transfer heat to the walls of the box.
2) As this happens, means that the gas molecules lose heat energy and is cooled.
3) KInetic energy is directly related with the temperature: therefore, the lower the temperature the lower the kinetic energy.
4) Kinetic energy is, also, directly related to the square of the speed. So, the lower the kinetic energy, the lower the average speed of the molecules.
That is why it is correct to say that the gas molecules slow down.
5) Since it is said that the walls of the box are very cold, it may well happen that the energy passed from the molecules to the walls of the box is enough to produce the phase change, from gas to liquid.
As for the other options:
i) the gas molecules speed up due to collisions with the walls: No. As already stated the gas molecules slow down.
ii) the gas molecules vibrate around fixed positions: No, the gas molecules are not around fixed positions, they translate in right random motion.
iii) the gas molecules gain energy from the cold walls of the box: as already stated the gas molecules release energy which is transfered to the cold walls of the box.
