Final answer:
In a solution, like salt in water, the solute dissolves evenly and forms a stable, homogeneous mixture. On the other hand, a suspension, like sand in water, contains particles that do not dissolve and can settle out or be filtered, comprising a heterogeneous mixture. The correct answer is A) Salt, sand.
Step-by-step explanation:
In a solution, a solute dissolves into a solvent evenly, such as salt in water; the solute, in this case salt, is almost uniformly distributed within the solvent, creating a homogeneous mixture. Solutions are stable, meaning that the solute does not settle out or separate by simple filtration. For example, when sugar is fully dissolved into water, it forms a stable solution that does not change if it stands or even if it goes through a filter, because the sugar particles are very small, typically less than 1 nm in diameter.
In contrast, a suspension consists of particles that are so large or complex that they remain undissolved and can be separated by filtration. An example of a suspension is sand in water, where particles are typically greater than 1000 nm and can settle upon standing. Suspensions are considered heterogeneous mixtures.
Given these definitions, the correct answer to the student's question is A) Salt, sand.