Which one of the following is the correct electronic configuration of sodium?

$(a)$ Which of the following electronic configurations are not possible? Give reasons.
$(i)$ $X: 2, 8, 4$
$(ii)$ $Y: 3, 8, 2$
$(iii)$ $Z: 2, 8, 9$
$(b)$ Write electronic configurations of the following elements and predict their valencies:
Fluorine: $9$,Aluminium: $13$,Argon: $18$

If $Z = 3$,what would be the valency of the element? Also,name the element.

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.

Find out the valency of an element $X$ having atomic number $16$.

$Na^{+}$ has completely filled $K$ and $L$ shells. Explain.