Final answer:
The physical property used to distinguish between saltwater and freshwater is density, because saltwater has a higher concentration of dissolved salts increasing its density over freshwater.
Step-by-step explanation:
When comparing a body of saltwater and a body of freshwater, the physical property you can use to distinguish between the two is density. Saltwater contains dissolved salts, mostly sodium chloride, which increase its density compared to freshwater. This is why saltwater tends to be denser and objects float more easily in it than in freshwater. The characteristic properties of matter, such as density, do not change the chemical composition of a substance and can be observed without altering the substance itself.
Therefore, when presented with the options of magnetism, color change, corrosive, and density, the correct answer is density. It is a consistent physical property that can be easily measured and compared between various bodies of water, making it a reliable indicator of saltwater versus freshwater.