(B) The electronic configuration of $Mg$ $(Z=12)$ is $[Ne] 3s^2$,which is a fully filled stable orbital configuration.The electronic configuration of

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(B) The electronic configuration of $Mg$ $(Z=12)$ is $[Ne] 3s^2$,which is a fully filled stable orbital configuration.The electronic configuration of $Al$ $(Z=13)$ is $[Ne] 3s^2 3p^1$.Due to the extra stability of the fully filled $3s$ subshell in $Mg$,it requires more energy to remove an electron compared to $Al$.Therefore,the first $I.P.$ of $Mg$ is more than $Al$.