# If $n=6$, the correct sequence for filling of electrons will be
$\begin{array}{1 1} (a)\;ns \to (n-1)d \to (n-2)f \to np \\ (b)\;ns - (n-2)f \to np \to (n-1)d \\ (c)\;ns - np \to (n-1)d \to (n-2)f \\ (d)\;ns \to (n-2)f \to (n-1)d \to np \end{array}$