Which of the following expressions is correct for CSCI unit with lattice parameter a?

Question

$\begin{array}{1 1} r_{cs}^++r_{cl}^-=2a \\ r_{cs}^++r_{cl}^-=\frac{a}{\sqrt 2} \\ r_{cs}^++r_{cl}^-=\frac{\sqrt 3}{2} a \\ none\;of \;the\;above \end{array}$