If NaCl is doped with $10^{-3}$ mole percentage of $SrCl_2$,what is the concentration of cation vacancy?

Question

$\begin{array}{1 1}6.023\times 10^{18}\\5.023\times 10^{18}\\6.023\times 10^{15}\\4.023\times 10^{18}\end{array}$