The electronic configuration of $\text{Fe}$ ($Z=26$) is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2$. The electronic configuration of $\text{Fe}^{2+}$ is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6$. Thus, the number of $d$-electrons in $\text{Fe}^{2+} = 6$. Now let's check each option:

• $\text{Mg}$ ($Z=12$): $1s^2 2s^2 2p^6 3s^2$. Number of $s$-electrons = $2 + 2 + 2 = 6$.
• $\text{Cl}$ ($Z=17$): $1s^2 2s^2 2p^6 3s^2 3p^5$. Number of $p$-electrons = $6 + 5 = 11$.
• $\text{Fe}$ ($Z=26$): Number of $d$-electrons = 6.
• $\text{Ne}$ ($Z=10$): $1s^2 2s^2 2p^6$. Number of $p$-electrons = 6. Thus, the number of $p$-electrons in $\text{Cl}$ (11) is different from the number of $d$-electrons in $\text{Fe}^{2+}$ (6).