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# The electronic configuration $1s^2,2s^2p^6,3s^13p^1$ correctly describes :

$\begin{array}{1 1}(a)\;Ground\;state \;of\;Na\\(b)\;Ground\;state\;of\;Si^+\\(c)\;Excited\;state\;of\;Mg\\(d)\;Excited\;state\;of\;Al^{3+}\end{array}$

Ground state of Na is $1s^2 \;2s^2\; 2p^6 \;3s^1$
Ground state of $Si^+$ is $1s^2 \;2s^2\; 2p^6 \;3s^2 \;3p^2$
Excited state of $Mg$ is $1s^2 \;2s^2\; 2p^6 \;3s^1\; 3p^1$
Excited state of $Al^{3+}$ is $1s^2\; 2s^2 \;2p^6$
Hence (c) is the correct answer.
edited Apr 2, 2014