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A pure mixture is best defined as two or more substances that retain their identities. In this context, when substances are combined to form a mixture, they do not undergo any chemical change or transformation; rather, each retains its own properties. This means that you can still distinguish the individual components in a pure mixture, such as oil and water, where each component clearly maintains its own characteristics.

The concept of identity retention is fundamental in chemistry. It emphasizes that, regardless of the physical state of the mixture—whether solid, liquid, or gas—the original substances are not altered on a molecular level. This forms the basis for understanding various types of mixtures and their distinction from pure substances, which have a uniform and definite composition.

In contrast, other definitions mentioned in the choices involve aspects such as chemical changes, which do not apply to pure mixtures, as well as ideas that imply homogeneity or separability that do not align with the fundamental characteristics of a mixture. Hence, the emphasis on the distinct and unchanged identities of the substances involved is crucial in defining a pure mixture accurately.