Which of the following is a preparation that must be shaken well and involves undissolved particles in a liquid?

The correct answer is a preparation known as a suspension. A suspension is a heterogeneous mixture where solid particles are dispersed throughout a liquid but are not dissolved in it. This means that the solid particles are large enough to eventually settle at the bottom if the mixture is left undisturbed, leading to a separation of components. To ensure an even distribution of the solid particles, suspensions must be shaken well before administration or use.

In contrast, a solution consists of solute particles that are completely dissolved in a solvent, resulting in a clear and stable mixture that does not require shaking. An emulsion is a mixture of two immiscible liquids, often stabilized by an emulsifying agent, and while it may require shaking to maintain its uniformity, it does not contain undissolved particles like a suspension. An elixir is a sweetened liquid containing alcohol and is typically used for medicinal purposes, but it also does not contain undissolved particles that would necessitate shaking. Therefore, suspensions are uniquely characterized by the presence of undissolved particles and the need for thorough mixing before use.