What is the BEST indication that the pre-treatment filter (or filters) of a reverse-osmosis freshwater generator must be serviced by cleaning or replacement?

Correct Answer: Option A — An excessive pressure drop across the pre-treatment filter.

Pre-treatment filters protect membranes from suspended solids and debris. An excessive pressure drop across these filters indicates they are becoming clogged and reaching their capacity. Servicing them promptly prevents flow restriction and protects the downstream high-pressure pump and membrane modules from potential damage or premature fouling.