ANS:A - free expansion of an ideal gas.

The thermodynamic law constantPV^y=constant is not followed by the free expansion of an ideal gas. This law, known as Boyle's Law, applies specifically to isothermal processes, where temperature remains constant. In the case of free expansion of an ideal gas, no external work is done on or by the gas, and there is no heat transfer, making it an adiabatic and isothermal process. Therefore, the gas undergoes a change in volume without any change in pressure, contrary to Boyle's Law.