If NaCl is doped with $10^{–3}$ mol % of $SrCl_2$, what is the concentration of cation vacancies?

Question

$\begin{array}{1 1}6.02\times 10^{18}\\5.02\times 10^{18}\\3.02\times 10^{18}\\4.02\times 10^{18}\end{array}$