= Solution
For a small radial comoving interval, $dD_C/dz=c/H(z)$. At $z=2$,
$$
H(2)=70\sqrt{0.75+0.25(3)^3}=191.7\ \mathrm{km\,s^{-1}\,Mpc^{-1}}.
$$
Thus
$$
\boxed{\Delta z\simeq\frac{H(2)\ell_c}{c}=0.0639}.
$$
This uses the unbound cosmological radial separation between separate <quasars>, unlike the peculiar-velocity <redshift> dispersion of galaxies inside the bound group. Integrating the distance relation instead of linearizing gives a small correction for a specified direction and endpoint. The quoted characteristic spacing alone does not assert that the same nearest object lies both purely transverse and purely radial.
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