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In the ground state an element has 13 electrons in M shell.The element is

$\begin{array}{1 1}(a)\;Copper&(b)\;Chromium\\(c)\;Nickel&(d)\;Iron\end{array}$

M shell=3rd shell
Electronic configuration =$1s^2,2s^2,2p^6,3s^2,3p^6,4s^1,3d^5$
No of electron in 3rd shell =$3s^2,3p^6,3d^5$
$\Rightarrow 2+6+5=13$
$e^-=24$ in Cr
Hence (b) is the correct answer.