If 'a' stands for the edge length of the cubic systems: simple cubic, body centred cubic and face centred cubic, then the ratio of the radii of the spheres in these systems will be respectively:
$\dfrac{1}{2}a : \dfrac{\sqrt{3}}{4}a : \dfrac{1}{2\sqrt{2}}a$
$\dfrac{1}{2}a : \sqrt{3}a : \dfrac{1}{\sqrt{2}}a$
$\dfrac{1}{2}a : \dfrac{\sqrt{3}}{2}a : \dfrac{\sqrt{2}}{2}a$
$1a : \sqrt{3}a : \sqrt{2}a$