Which of the following is a pure substance?

A pure substance is defined as a material that has a consistent composition and distinct chemical properties. Among the choices provided, an element, an atom, and a compound molecule fall under the category of pure substances. An element represents the simplest form of matter and cannot be broken down into simpler substances by chemical means. An atom is the basic building block of matter and is a pure substance in itself. A compound, which is a molecule formed from two or more different elements bonded together in fixed proportions, is also classified as a pure substance due to its uniform chemical composition.

In contrast, the other options refer to mixtures or categories that do not guarantee purity. A mixture of two or more elements does not qualify as a pure substance because its composition can vary and does not have fixed ratios. Similarly, a mixture of compounds would also be inconsistent in composition, making it a mixture rather than a single pure substance. Lastly, stating "all organic compounds" does not guarantee purity either, as many organic compounds can exist as mixtures or can vary widely in their compositions depending on the context. Thus, option A represents pure substances accurately, while the others describe mixtures or general categories that do not define unique chemical identities.