Assertion (A): Carbon dioxide can be liquefied by the application of pressure equal to 73 atmospheres at temperature above 31 °C. Reason (R): A gas can be liquefied by the application of critical pressure when it is cooled below its critical temperature.

Assertion (A) is false because a gas cannot be liquefied by pressure alone if its temperature is above its critical temperature. For carbon dioxide, the critical temperature is approximately 31.1 °C and the critical pressure is 73.8 bar (approx. 73 atm) [t1][t2][t6]. Above 31.1 °C, CO2 exists as a supercritical fluid, where the distinct liquid and gas phases disappear, and increasing pressure will not result in liquefaction [t3][t8]. Reason (R) is true as it correctly defines the principle of liquefaction: a gas can be converted into a liquid by applying pressure only when it is cooled below its critical temperature [t3][t6]. In the region below the critical temperature, the vapor-liquid line exists, allowing phase transition through compression [t3]. Since A is false and R is true, option 4 is the correct choice.