Which of the following gases is most difficult to liquefy?

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

The temperature below which a gas cannot exist is 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?

Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical temperatures of ammonia and carbon dioxide are $405.5 \ K$ and $304.10 \ K$ respectively. Which of these gases will liquefy first when you start cooling from $500 \ K$ to their critical temperature?

Isotherms of carbon dioxide at various temperatures are represented in the figure. Answer the following questions based on this figure.
$(i)$ In which state will $CO_2$ exist between the points $a$ and $b$ at temperature $T_1$?
$(ii)$ At what point will $CO_2$ start liquefying when temperature is $T_1$?
$(iii)$ At what point will $CO_2$ be completely liquefied when temperature is $T_2$?
$(iv)$ Will condensation take place when the temperature is $T_3$?
$(v)$ What portion of the isotherm at $T_1$ represents liquid and gaseous $CO_2$ at equilibrium?