Final answer:
Real gases act most ideally at low pressure and high temperature. High pressures and low temperatures lead to significant deviations from ideal behavior.
Step-by-step explanation:
Real gases behave most ideally at low pressure and high temperature. This is due to the kinetic-molecular theory assumptions that gas molecules have negligible volume and experience negligible intermolecular interactions, which are more closely approximated under these conditions.
At high pressures and low temperatures, real gases deviate significantly from ideal behavior as molecule volume and intermolecular forces cannot be ignored. Particularly, the occurrence of liquefaction at low temperatures indicates a substantial departure from the ideal gas behavior.