Which of the following best describes a colloid?

A colloid is characterized as a heterogeneous mixture where fine particles are dispersed throughout a continuous medium but do not settle out over time. This property distinguishes colloids from solutions and suspensions. In colloids, the particle size is typically between one nanometer to one micrometer, which allows them to remain suspended in the medium, resulting in a stable mixture that exhibits properties such as the Tyndall effect, where light is scattered by the dispersed particles.

In comparison, a heterogeneous mixture that allows particles to settle out over time does not retain the stability that is essential to colloids. A homogeneous mixture refers to a solution where components are evenly distributed, but this definition does not capture the unique characteristics of colloids. Lastly, a pure substance is uniform throughout and does not represent a mixture, making it distinctly different from colloids, which inherently involve multiple components. Thus, the description of a colloid as a heterogeneous mixture with fine particles that remain dispersed is accurate and emphasizes its unique properties.