= Solution
For each spacing pair, the largest geometrical period occurs with parallel projected reciprocal vectors. The closest pair is $d_{\rm Cu}=0.208\,\mathrm{nm}$ and $d_{\rm substrate}=0.212\,\mathrm{nm}$, giving
$$
\boxed{D_{\max}=\frac{0.208\times0.212}{0.212-0.208}\,\mathrm{nm}=11.024\,\mathrm{nm}\simeq11.0\,\mathrm{nm}.}
$$
However, the <crystal> is only $10\,\mathrm{nm}$ across. It contains at most $10/11.024\simeq0.91$ of this period, so \b[the largest formal period cannot appear as a repeated fringe train inside one <crystal>]. It can instead give a broad variation of contrast. A period of order the particle diameter is the largest directly sampled scale, and even that is poorly constrained by one particle.
The distinction matters if “see” means a complete repeat. Of the exactly parallel spacing pairs, the largest other period is approximately $2.09\,\mathrm{nm}$ from the $0.208$ and $0.231\,\mathrm{nm}$ pair. If relative <rotation> is continuously variable, the closest pair can instead be rotated slightly to shorten its period below $10\,\mathrm{nm}$, so $2.09\,\mathrm{nm}$ is not a universal maximum visible period for arbitrary orientations.
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