The correct increasing order of the boiling points of liquid oxygen, liquid argon and liquid nitrogen present in liquid air is :
Liquid nitrogen has boiling point equal to - 190⁰C and thus turns into gas first and separated from air. The boiling point of argon is - 186⁰C, therefore it is extracted after nitrogen. The boiling point of oxygen is - 183⁰C, thus it is collected after the extraction of argon.