Mixtures are broadly classified into two types based on the uniformity of their composition: homogeneous mixtures and heterogeneous mixtures.

A homogeneous mixture is one in which the components are mixed so thoroughly, right down to the molecular or ionic level, that the composition and properties (such as density, colour, and taste) are exactly the same throughout every part of the sample — a single phase is observed, and no boundary between the constituents can be detected even under a microscope. Common examples include saltwater, sugar dissolved in water, air (a mixture of gases), and alloys such as brass (a solid solution of copper and zinc).

A heterogeneous mixture, by contrast, has a non-uniform composition — different regions of the sample can have different proportions of the components, and distinct phases or visible boundaries are present. Examples include sand mixed with water, oil floating on water, and a salad.

Given this distinction, the statement that correctly describes a homogeneous mixture is that it has uniform composition throughout.

The remaining statements describe features of heterogeneous mixtures, not homogeneous ones, or are simply factually inaccurate about mixtures in general:

• Forming separate layers on standing indicates that the components are not fully miscible and are separating out under gravity — a classic sign of a heterogeneous mixture (e.g., oil and water), not a homogeneous one.
• Containing only solid components is not a requirement at all — homogeneous mixtures can be gaseous (air), liquid (saltwater), or even solid (alloys); the phase of matter is unrelated to whether a mixture is homogeneous.
• Having visible particles that settle on standing is a hallmark of a heterogeneous mixture (specifically, a suspension), since true homogeneous mixtures have particle sizes small enough (typically at the atomic, ionic, or molecular level) that they never settle out under gravity.