In which type of mixture do particles remain suspended indefinitely?

The correct choice, a colloid, is characterized by its ability to maintain particles in suspension indefinitely. In a colloid, the particle sizes range between 1 nanometer and 1 micrometer, which prevents them from settling to the bottom due to gravity. This stability is largely due to the interactions between the dispersed particles and the medium in which they are suspended.

Because of the relatively small size of the particles and their interactions with the solvent molecules, they remain evenly distributed throughout the mixture without precipitating out. Examples of colloids include milk, where fat globules are dispersed in water, and fog, where tiny water droplets are suspended in air.

In contrast, a solution consists of solute particles that are at the molecular or ionic level, which dissolve completely and do not settle or scatter light, exhibiting transparency. In a suspension, the particles are larger and can settle out over time when left undisturbed. Emulsions, which are a type of colloid, consist specifically of two immiscible liquids where one is dispersed in the other, but they can separate unless an emulsifying agent is used. Thus, while emulsions share characteristics with colloids, the definition of colloids itself is the most accurate fit for the given question