The correct answer is C) An excessive pressure drop across the membrane modules. The reasoning is that an excessive pressure drop across the reverse osmosis membrane modules indicates that the membranes have become fouled or scaled, which requires servicing by flushing or cleaning. As the membranes become fouled, the resistance to water flow increases, resulting in a higher pressure drop across the membrane modules. The other options are incorrect because a lower than normal feed pressure (A) may indicate other issues in the system, and an insufficient pressure drop (B) or an excessive pressure drop across the pre-treatment filter (D) do not necessarily indicate the need to service the membrane modules.