To determine which gas is most easily liquefied, we need to consider the van der Waals constant $a. \\$The constant $a$ is a measure of the attraction between particles.$\\$A higher value of $a$ indicates stronger intermolecular forces, which makes the gas easier to liquefy.$\\$Given van der Waals constants:$\\$• $a_{NH_3} = 4.17 \\$• $a_{H_2} = 0.244 \\$• $a_{O_2} = 1.36 \\$• $a_{CO_2} = 3.59 \\$Comparing these values, we see that $a_{NH_3}$ is the highest.$\\$Therefore, ammonia (NH$_3$) has the strongest intermolecular forces among the given gases.$\\$This means NH$_3$ is the most easily liquefied gas.$\\$Thus, the correct option is Option 3: NH$_3.$