The correct answer is B) Modules may be piped in series to increase the fresh water purity. In a reverse osmosis (RO) system with multiple spiral-wound membrane modules, connecting the modules in series allows the water to pass through multiple stages of filtration, effectively increasing the purity of the final freshwater output. This is because each successive module removes additional impurities and dissolved solids, resulting in a higher quality of freshwater. The other options are incorrect: A) is incorrect because connecting modules in series increases purity, not throughput; C) is incorrect because connecting modules in parallel would increase throughput, not purity; and D) is incorrect because connecting modules in series would not decrease the likelihood of fouling, as each module would still be subject to the same fouling potential.