# An ion with 18 electrons in the outermost shell is :

$\begin{array}{1 1}(a)\;K^+&(b)\;Cu^+\\(c)\;Cs^+&(d)\;Th^{4+}\end{array}$

$Cu^+$ has $1s^2,2s^22p^6,3s^23p^63d^{10}$ configuration.
Hence (b) is the correct answer.