A) $1 s\, 2 s$

B) $1 s ^2 \,2 s ^2 \,2 p$

C) $1 s^2\, 2 s^2 \,2 p^5$

D) $1 s ^2 \,2 s ^2 2 p^6$

From the options above we notice that the greatest energy will be involved for option $C$ as it has attained the noble gas configuration after the loss of $2$ electrons and removal of a $3$ electron will require a lot of energy.