Which of the following gases is most difficult to liquefy?

The temperature and pressure at which ice,liquid water,and water vapour can exist together are:

Explain how liquefaction can occur in different steps within the same phase using an example.

The gas with the highest critical temperature is

What are the most favourable conditions to liquefy a gas?

Four gas cylinders containing $He$,$N_2$,$CO_2$ and $NH_3$ gases separately are gradually cooled from a temperature of $500 \ K$. Which gas will liquefy first?
(Given $T_C$ in $K - He: 5.3, N_2: 126, CO_2: 304.1$ and $NH_3: 405.5$)