Which of the following is an example of a suspension?

A suspension is a heterogeneous mixture in which solid particles are dispersed in a liquid or gas but are not dissolved. These particles are typically larger than those found in solutions and can eventually settle at the bottom if left undisturbed.

In the case of sand in water, the sand particles remain suspended for a certain period but will eventually settle to the bottom if the water is still. This characteristic defines a suspension. Unlike in true solutions, where solute particles are completely dissolved and cannot be seen, the solid particles in a suspension are visible and can be separated through filtration or settling.

The other choices represent different types of mixtures. A saltwater solution is a homogeneous mixture where salt is completely dissolved in water, resulting in a clear liquid. Milk is an emulsion, which is a specific type of colloidal suspension that contains fat globules dispersed in water. Vinegar is a solution of acetic acid in water, thus also being homogeneous. None of these options exhibit the settling property and visible solid particles found in a suspension, making the choice of sand in water an appropriate example of a suspension.