ANS:C - do not exert attractive force on each other.

The unrestrained expansion of an ideal gas does not result in its cooling due to the reason that the gas molecules do not exert attractive forces on each other. In an ideal gas, molecules are assumed to have no intermolecular forces (except during collisions), meaning they do not exert attractive forces on each other. When an ideal gas expands into a vacuum or against a piston without doing external work, there is no energy transfer in the form of work being done by or against attractive forces. As a result, there is no change in the average kinetic energy of the gas molecules, and therefore no cooling occurs. Therefore, the statement "do not exert attractive force on each other" is the reason why unrestrained expansion of an ideal gas does not result in its cooling.