Concept:A mixture is a substance made of two or more constituents that can be separated by physical methods.Explanation:Granulated sugar, sodium chloride, and iron filings are pure substances with fixed compositions.Seawater contains water, dissolved salts, and other materials.These components in seawater can be separated by physical methods such as evaporation or filtration.Therefore, seawater is a mixture.Answer:B. Seawater