Above what temperature can a gas not be liquefied by pressure alone, no matter how much pressure is applied?

**Critical temperature $T_c$** is the highest temperature at which a substance can exist as a liquid. Above $T_c$, no amount of compression will liquefy the gas — the gas and liquid phases become indistinguishable (called a supercritical fluid). Examples: $\mathrm{CO_2}$ has $T_c = 31°$C, so it can be liquefied at room temperature with pressure. Oxygen has $T_c = -119°$C, so it cannot be liquefied just by pressing at room temperature. You must cool it first. **Boyle temperature** (option A) is something else: the $T$ at which a real gas behaves most ideally over a range of pressures.