Which law states absolute zero cannot be reached?

This would be a consequence of the 2nd law and the "zeroeth" law. Heat always flows from the warmer object to the colder one (2nd law), so to cool something down to absolute zero, you would have to have something colder than absolute zero to absorb the heat coming out. The zeroeth law, however, defines absolute zero as a ground state where energy has been reduced to its absolute minimum

• thus you cannot go below absolute zero and thus there cannot be

something to absorb the energy to take an object to system down to absolute zero.