I'm pretty sure it's D. Compounds
Salt water is an example of a solution, and a solution is a type of mixture, so by multiple choice logic none can be the right answer! Seriously though - mixture are combinations of two compounds that are physically combined but do not undergo a chemical reaction with one another to produce a new compound. For instance, when you dissolve salt in water, you don't produce any new products, and you can boil away the water to leave only the salt (a physical method of separation). You cannot separate a compound by physical means because by definition this would be a chemical reaction as you're breaking bonds.
