The correct answer is A) Conductance of water. A salinometer measures the salinity of water by determining its electrical conductance. The conductance of water is directly related to the concentration of dissolved salts, which is what defines salinity. As the salinity of the water increases, its ability to conduct electricity also increases. By measuring the electrical conductance of a water sample, a salinometer can accurately determine the salinity. The other answer choices are incorrect because they do not represent the operating principle of a salinometer. Specific gravity, optical transparency, and capacitance are not the primary factors used to determine salinity using a salinometer.