What is the BEST indication that the membrane modules of a reverse-osmosis freshwater generator must be serviced by flushing or cleaning?

Correct Answer: Option C — An excessive pressure drop across the membrane modules.

Excessive pressure drop across membrane modules indicates fouling or scaling, which restricts flow and reduces efficiency. Monitoring differential pressure is a critical diagnostic tool for reverse-osmosis systems. Per marine engineering best practices, this increase necessitates immediate chemical cleaning or flushing to restore design flux and prevent permanent membrane damage.