Which type of dispersion has a uniform composition but varies in particle size?

The correct answer is based on the definition and characteristics of colloidal dispersions. A colloidal dispersion consists of particles that are dispersed throughout a continuous phase, which can be liquid, solid, or gas. In colloids, the composition appears uniform at a macroscopic level, but the size of the dispersed particles is typically larger than molecules and can vary in size.

Colloidal systems can include mixtures where larger particles are stabilized within a solution, yet they do not settle out like those in suspensions. This characteristic enables a colloidal system to maintain a relatively uniform composition, even though the individual particles differ in size. Classical examples of colloids include milk, fog, and smoke, where the dispersed phase is present in varying sizes without altering the overall appearance of uniformity.

Other options, such as true solutions, have a consistent particle size at the molecular level, while suspensions consist of larger particles that may not remain evenly dispersed and can settle out over time. Foams involve gas bubbles dispersed within a liquid or solid matrix but do not specifically refer to the variation of particle size characteristic of colloidal dispersions. Thus, among the given options, the colloidal category aptly fits the description of having a uniform composition alongside varying particle sizes.