The correct answer is D) location of salt water contamination. A salinity indicator is used to detect the presence and measure the concentration of dissolved salts in water. This is particularly important in the marine environment, where the detection of saltwater contamination is crucial for maintaining the proper operation of marine equipment and systems. By using a salinity indicator, the location of salt water contamination can be identified, allowing for appropriate remedial actions to be taken. The other answer choices are incorrect because they do not accurately describe the primary purpose of a salinity indicator. A salinity indicator is not used to determine the level of alkalinity in condensate (A), the chemical makeup of feed water (B), or the cause of salt contamination (C).