Which type of substance is composed of molecules that contain two or more different kinds of atoms?

The correct choice is based on the definition of compounds in chemistry. A compound is a type of pure substance that is formed when two or more different elements chemically bond together. These elements combine in fixed proportions to create a new substance with distinct properties that are different from those of the individual elements.

For example, water (H₂O) is a compound made up of two hydrogen atoms and one oxygen atom. The combination of these different atoms results in a substance that has unique characteristics, such as being liquid at room temperature, which differs from the gaseous state of oxygen and the gaseous state of hydrogen at the same conditions.

The other types of substances mentioned do not fully fit this description. Pure substances can consist of either elements or compounds, but they do not specifically denote the presence of different types of atoms. An element consists of only one type of atom and cannot be broken down into simpler substances by chemical means. A mixture, on the other hand, is a physical combination of two or more pure substances, which retain their individual properties and can vary in composition. Thus, they do not necessarily contain different kinds of atoms bonded together in a fixed ratio, as seen in compounds.