How does the strength of the charge-charge interaction between two ions depend on the separation in the absence of any other ions?

Question

$\begin{array}{1 1}(a)\;\propto \large\frac{1}{r^6}&(b)\; \propto \large\frac{1}{r^3}\\(c)\;\propto \large\frac{1}{r^2}&(d)\;\propto \large\frac{1}{r}\end{array}$