Which of the following conditions is most favorable for the liquefaction of a gas?

Why does the boundary between the liquid phase and the gaseous phase disappear upon heating a liquid up to the critical temperature in a closed vessel? In this situation,what will be the state of the substance?

Amongst the following statements,the correct one is

$A$ gas can be liquefied:

What is the critical temperature $T_C$? What is its value for $CO_2$?

Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the particles. Following are the critical temperatures of some gases:

$Gases$	$Critical \ temperature \ in \ Kelvin$
$P$	$33.2$
$Q$	$5.3$
$R$	$154.3$
$S$	$126$

From the above data,what would be the order of liquefaction of these gases? Start writing the order from the gas liquefying first.