The correct answer is B) Modules may be piped in series to increase the freshwater purity. The reasoning is that in a reverse-osmosis freshwater generator, piping the modules in series increases the number of filtration stages, which in turn improves the purity of the final freshwater output. This is because each additional module filters out more impurities, resulting in a higher quality freshwater product. The other options are incorrect because: A) piping in series increases throughput, not purity; C) piping in parallel increases throughput, not purity; and D) piping in series does not decrease the likelihood of fouling, which is mainly influenced by pretreatment and cleaning processes.