To determine which option does not agree with the variation of the property indicated, we need to analyze each option separately.$\\$Option 1: $Al^{3+} < Mg^{2+} < Na^+ < F^-$ (increasing ionic size)$\\$• Ionic size generally increases with decreasing positive charge and increasing negative charge.$\\$• $Al^{3+}$ has the smallest size due to the highest positive charge.$\\$• $Mg^{2+}$ is smaller than $Na^+$ because of the higher positive charge.$\\$• $Na^+$ is smaller than $F^-$ because $F^-$ has a negative charge.$\\$• This order is correct.$\\$Option 2: $B < C < N < O$ (increasing first ionization enthalpy)$\\$• First ionization enthalpy generally increases across a period from left to right.$\\$• However, nitrogen has a higher ionization enthalpy than oxygen due to its half-filled p orbital stability.$\\$• The correct order should be $B < C < O < N. \\$• This order is incorrect.$\\$Option 3: $I < Br < Cl < F$ (increasing electron gain enthalpy)$\\$• Electron gain enthalpy becomes more negative as we move up the group in halogens.$\\$• Fluorine has less negative electron gain enthalpy than chlorine due to its small size and high electron-electron repulsion.$\\$• The correct order is $I < Br < F < Cl. \\$• This order is incorrect.$\\$Option 4: $Li < Na < K < Rb$ (increasing metallic radius)$\\$• Metallic radius increases down the group.$\\$• This order is correct.$\\$Therefore, the option that does not agree with the variation of property indicated is Option 2.