Which of the following is a characteristic of an alloy?

An alloy is defined as a mixture of two or more elements, where at least one of the components is a metal. A key characteristic of alloys is their uniform composition, meaning that the different elements are mixed together in such a way that they appear homogeneous at the macro or microscopic level. This uniformity often results in enhanced physical and chemical properties compared to the individual components, making alloys widely used in various applications.

In terms of the other choices: an alloy typically contains multiple types of metals or other elements rather than just one type; it can be melted, which is one of the processes used to create or modify alloys; and while some alloys may include non-metal components, it is not a requirement for all alloys. The defining trait that makes "a uniform composition" the correct answer highlights the consistent structure that contributes to the overall properties of the material.