Final answer:
A compound is a pure substance with a fixed composition and set of properties. Mixtures and solutions, consisting of two or more substances combined, are not pure substances as their properties can vary.
Step-by-step explanation:
When determining whether a substance is pure or a mixture, it's essential to understand their differences. A pure substance is one that has a constant composition and properties throughout the sample. This can be either an element, which consists of a single type of atom, or a compound, which is made up of two or more different atoms chemically bonded together. In contrast, mixtures consist of two or more pure substances physically combined, and these can be further classified as homogeneous (solutions) or heterogeneous mixtures. Homogeneous mixtures have uniform composition throughout, whereas heterogeneous mixtures do not.
Thus, when looking at the options provided, a compound is a pure substance as it has a fixed composition and set of properties. Mixtures and solutions, on the other hand, are not pure substances as they combine different substances whose properties can vary depending on the composition.