The number of d-electrons in Fe²⁺ (Z = 26) is not equal to the number of electrons in which one of the following?

$\begin{array}{l}{ }_{26}{\mathrm{Fe}}^{2+}:1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^6,3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6\\ { }_{17}\mathrm{Cl}:1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^6,3{\mathrm{s}}^{2}3{\mathrm{p}}^5\\ { }_{26}\mathrm{Fe}:1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^6,3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6,4{\mathrm{s}}^2\\ { }_{10}\mathrm{Ne}:1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^6\\ { }_{12}\mathrm{Mg}:1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^6,3{\mathrm{s}}^2\end{array}$