Which of the following gases is most difficult to liquefy?

What happens when the temperature of $CO_2$ at its critical temperature of $30.98 \, ^\circ C$ is increased or decreased?

The isotherms of a gas are shown below:
Among the following:
$(i)$ At $T_1$,the gas cannot be liquefied.
$(ii)$ At point $B$,liquid starts to appear at $T_2$.
$(iii)$ $T_c$ is the highest temperature at which the gas can be liquefied.
$(iv)$ At point $A$,a small increase in pressure condenses the whole system to a liquid.
The correct statements are:

At which of the following conditions can a gas be liquefied? ($T_c$ and $P_c$ are critical temperature and pressure.)

Critical temperature for carbon dioxide and methane are $31.1^{\circ} C$ and $-81.9^{\circ} C$ respectively. Which of these has stronger intermolecular forces and why?

What is meant by the vapor of a substance like $CO_2$?