Answer is (1) - boiling point.
After liquefying the nitrogen and oxygen gases, they are in same mixture of liquid phase. Hence, to separate those gases we can use their boiling points. Normal method is after liquefying the gases, slowly start to increase the temperature of the mixture. Then, the gases will start to go its gaseous phase when the temperature reachs to their boiling points. In this way, we can separate those gases. But to get pure gas, there should be a significant difference between those boiling points.