Final answer:
The answer to which of the following is a mixture is D. salt and sugar because when combined, they form a heterogeneous mixture where each component retains its own properties.
Step-by-step explanation:
The question asks which of the following is a mixture: A. water B. sugar C. salt D. salt and sugar. To answer this, we need to understand the difference between pure substances and mixtures. Water, sugar (sucrose), and salt (sodium chloride) are all pure substances because they consist of one type of molecule with a consistent chemical composition. In contrast, if you mix salt and sugar together, they do not chemically combine to form a new substance; instead, they retain their individual properties, creating a heterogeneous mixture where you can still differentiate between the salt and sugar particles if you look closely enough. Therefore, the answer is D. salt and sugar.