The correct answer is C) High fresh water outlet salinity. This is the correct answer because if the membrane module is damaged and no longer semi-permeable, it will allow the solute (salt) to pass through more easily, resulting in higher than normal salinity in the fresh water outlet. This indicates that the membrane is not effectively removing the salts from the seawater, which is the primary function of the reverse osmosis system. The other options are incorrect because: A) Lower than normal fresh water production could be caused by other factors like fouling or scaling, not necessarily membrane damage; B) Higher than normal brine overboard back pressure may indicate other issues like clogging, not membrane damage; and D) Low fresh water outlet salinity would indicate the opposite of the problem described in the question.