Which among the following gas is stored in liquid form?

Storage Nitrous oxide is stored as a liquid in French blue cylinders with a gauge pressure of 51 bar at 20?C, which therefore bears no correlation to cylinder content until all remaining N2O is in the gaseous phase. N2O is usually stored below its critical temperature and so it is in simultaneous liquid and vapor phases. Miscellaneous Anaesthetics Colour coding of Gas cylinders Colour coding of Gas cylinders (for UK and ISO) : Substance Gas/vapour Symbol Colour coding Cylinder Colour coding Shoulder Oxygen Gas O2 Black White Nitrous oxide Vapour N2O Blue Blue Entonox Gas and Vapour N2O/O2 Blue White/Blue Air Gas AIR Black White/Black Carbon dioxide Vapour CO2 Grey Grey Helium Gas He Brown Brown Ref: Fundamentals of Anaesthesia By Tim Smith, Colin Pinnock, Ted Lin, 3rd Edition, Page 830.