To determine the order of enthalpy of hydrogenation, we need to consider the stability of the compounds.$\\$Given structures:$\\$• Structure (I) is 1,2,3-trimethylbenzene (aromatic compound).$\\$• Structure (II) is 1,2,3-trimethylcyclohexene (non-aromatic with one double bond).$\\$• Structure (III) is 1,3,5-hexatriene (non-aromatic with three double bonds).$\\$The enthalpy of hydrogenation is inversely related to the stability of the compound.$\\$Aromatic compounds are more stable due to resonance, resulting in lower enthalpy of hydrogenation.