What is the BEST indication that the membrane module is damaged and is no longer semi-permeable and allowing the solute to pass through?

Correct Answer: Option C — High fresh water outlet salinity.

A semi-permeable membrane is designed to reject dissolved salts. If the membrane is physically damaged or torn, it loses its selectivity, allowing salt ions to pass directly into the permeate stream. Consequently, a sharp increase in the salinity of the freshwater outlet is the primary indicator of membrane failure.