= Solution
For a transverse \i[comoving] separation $\ell_c=100$ Mpc in a flat universe, the corresponding distance is the transverse <comoving distance> $D_M=D_C$, so
$$
\boxed{\Delta\theta\simeq\frac{\ell_c}{D_C}=\frac{100}{5421}=0.01845\ \mathrm{rad}=1.06^\circ}.
$$
Equivalently the physical separation is $100/3$ Mpc and dividing it by $D_A=5421/3$ Mpc gives the same angle. Dividing 100 comoving Mpc directly by $D_A$ would mix scale conventions. The density's $n_c^{-1/3}=100$ Mpc is a characteristic inter-object spacing; a particular nearest neighbour has a distribution of distances and orientations.
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