Kcrypon ($36_Kr)$ has the electronic configuration $(_{18}Ar)4s^23d^{10}4p^6$,the $37^{th}$ electron will go into which of the following subshells?

Question

$\begin{array}{1 1}(a)\;4f&(b)\;4d\\(c)\;6p&(d)\;5s\end{array}$