= Solution
Taking Cu K$\alpha$ <wavelength> $\lambda=0.15418\,\mathrm{nm}$, <Bragg's law> gives $d=\lambda/(2\sin\theta)$. For cubic $111$, $a=\sqrt3d_{111}$ and the nearest Zn–S separation is $\sqrt3a/4=3d_{111}/4$. For ideal <wurtzite structure>, $c=2d_{002}$ and the nearest separation is $3c/8=3d_{002}/4$. Therefore
$$
\boxed{r_{\rm cubic}=0.2346\,\mathrm{nm},\qquad r_{\rm hexagonal}\simeq0.2337\,\mathrm{nm}.}
$$
Both have tetrahedral nearest-neighbour <coordination number> four, but different further-neighbour stacking and lattice relaxation can shift their equilibrium bond lengths. The hexagonal value assumes ideal $c/a=\sqrt{8/3}$ and $u=3/8$; a real distorted <wurtzite structure> can have unequal axial and inclined bonds. One measured $002$ spacing alone does not determine every bond length without that structural assumption.
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