What is the temperature below which a gas cannot exist as a gas (i.e.,it liquefies) called?

Critical constants for a real gas are given as:
$T_C = 180 \ K$; $V_C = 0.123 \ L \ mol^{-1}$
$P_C = 45 \ atm$; $R = 0.082 \ L \ atm \ mol^{-1} \ K^{-1}$
Which of the following statements is correct for this real gas?

What are the most favourable conditions to liquefy a gas?

What is the surface tension at the critical temperature?

The critical temperature of $O_2$ is $154.3 \ K$ and the critical temperature of $N_2$ is $126.0 \ K$. Which gas will liquefy first during the liquefaction of air?

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