Final answer:
Option c) Salt water is a mixture because it contains at least two substances—salt and water—which are not chemically combined. Saltwater is a homogeneous mixture, with the salt evenly distributed in the water.
Step-by-step explanation:
The question seeks to identify which choice among the given options is a mixture. A mixture is a material composed of two or more different substances which are not chemically combined and can be separated by physical means. Among the options given, c) Salt water is a mixture.
Saltwater acts as if it were a single substance even though it contains at least two substances—salt, which is sodium chloride (NaCl), and water (H2O). Saltwater is considered a homogeneous mixture, or a solution, because the salt is evenly distributed throughout the water, and it forms a single phase where the composition is uniform throughout.
Potassium (a), Carbon Dioxide (b), and Calcium (d) are all pure substances, either elements or compounds, but not mixtures.